lttng lib: ring buffer

Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/lttng/lib/ringbuffer/api.h

+#ifndef _LINUX_RING_BUFFER_API_H
+#define _LINUX_RING_BUFFER_API_H
+
+/*
+ * linux/ringbuffer/api.h
+ *
+ * Copyright (C) 2010 - Mathieu Desnoyers "mathieu.desnoyers@efficios.com"
+ *
+ * Ring Buffer API.
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include "../../wrapper/ringbuffer/backend.h"
+#include "../../wrapper/ringbuffer/frontend.h"
+#include "../../wrapper/ringbuffer/vfs.h"
+
+/*
+ * ring_buffer_frontend_api.h contains static inline functions that depend on
+ * client static inlines. Hence the inclusion of this "api" header only
+ * within the client.
+ */
+#include "../../wrapper/ringbuffer/frontend_api.h"
+
+#endif /* _LINUX_RING_BUFFER_API_H */

drivers/staging/lttng/lib/ringbuffer/backend.h

+#ifndef _LINUX_RING_BUFFER_BACKEND_H
+#define _LINUX_RING_BUFFER_BACKEND_H
+
+/*
+ * linux/ringbuffer/backend.h
+ *
+ * Copyright (C) 2008-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring buffer backend (API).
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ *
+ * Credits to Steven Rostedt for proposing to use an extra-subbuffer owned by
+ * the reader in flight recorder mode.
+ */
+
+#include <linux/types.h>
+#include <linux/sched.h>
+#include <linux/timer.h>
+#include <linux/wait.h>
+#include <linux/poll.h>
+#include <linux/list.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+
+/* Internal helpers */
+#include "../../wrapper/ringbuffer/backend_internal.h"
+#include "../../wrapper/ringbuffer/frontend_internal.h"
+
+/* Ring buffer backend API */
+
+/* Ring buffer backend access (read/write) */
+
+extern size_t lib_ring_buffer_read(struct lib_ring_buffer_backend *bufb,
+				   size_t offset, void *dest, size_t len);
+
+extern int __lib_ring_buffer_copy_to_user(struct lib_ring_buffer_backend *bufb,
+					  size_t offset, void __user *dest,
+					  size_t len);
+
+extern int lib_ring_buffer_read_cstr(struct lib_ring_buffer_backend *bufb,
+				     size_t offset, void *dest, size_t len);
+
+extern struct page **
+lib_ring_buffer_read_get_page(struct lib_ring_buffer_backend *bufb, size_t offset,
+			      void ***virt);
+
+/*
+ * Return the address where a given offset is located.
+ * Should be used to get the current subbuffer header pointer. Given we know
+ * it's never on a page boundary, it's safe to write directly to this address,
+ * as long as the write is never bigger than a page size.
+ */
+extern void *
+lib_ring_buffer_offset_address(struct lib_ring_buffer_backend *bufb,
+			       size_t offset);
+extern void *
+lib_ring_buffer_read_offset_address(struct lib_ring_buffer_backend *bufb,
+				    size_t offset);
+
+/**
+ * lib_ring_buffer_write - write data to a buffer backend
+ * @config : ring buffer instance configuration
+ * @ctx: ring buffer context. (input arguments only)
+ * @src : source pointer to copy from
+ * @len : length of data to copy
+ *
+ * This function copies "len" bytes of data from a source pointer to a buffer
+ * backend, at the current context offset. This is more or less a buffer
+ * backend-specific memcpy() operation. Calls the slow path (_ring_buffer_write)
+ * if copy is crossing a page boundary.
+ */
+static inline
+void lib_ring_buffer_write(const struct lib_ring_buffer_config *config,
+			   struct lib_ring_buffer_ctx *ctx,
+			   const void *src, size_t len)
+{
+	struct lib_ring_buffer_backend *bufb = &ctx->buf->backend;
+	struct channel_backend *chanb = &ctx->chan->backend;
+	size_t sbidx, index;
+	size_t offset = ctx->buf_offset;
+	ssize_t pagecpy;
+	struct lib_ring_buffer_backend_pages *rpages;
+	unsigned long sb_bindex, id;
+
+	offset &= chanb->buf_size - 1;
+	sbidx = offset >> chanb->subbuf_size_order;
+	index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
+	pagecpy = min_t(size_t, len, (-offset) & ~PAGE_MASK);
+	id = bufb->buf_wsb[sbidx].id;
+	sb_bindex = subbuffer_id_get_index(config, id);
+	rpages = bufb->array[sb_bindex];
+	CHAN_WARN_ON(ctx->chan,
+		     config->mode == RING_BUFFER_OVERWRITE
+		     && subbuffer_id_is_noref(config, id));
+	if (likely(pagecpy == len))
+		lib_ring_buffer_do_copy(config,
+					rpages->p[index].virt
+					    + (offset & ~PAGE_MASK),
+					src, len);
+	else
+		_lib_ring_buffer_write(bufb, offset, src, len, 0);
+	ctx->buf_offset += len;
+}
+
+/**
+ * lib_ring_buffer_memset - write len bytes of c to a buffer backend
+ * @config : ring buffer instance configuration
+ * @bufb : ring buffer backend
+ * @offset : offset within the buffer
+ * @c : the byte to copy
+ * @len : number of bytes to copy
+ *
+ * This function writes "len" bytes of "c" to a buffer backend, at a specific
+ * offset. This is more or less a buffer backend-specific memset() operation.
+ * Calls the slow path (_ring_buffer_memset) if write is crossing a page
+ * boundary.
+ */
+static inline
+void lib_ring_buffer_memset(const struct lib_ring_buffer_config *config,
+			    struct lib_ring_buffer_ctx *ctx, int c, size_t len)
+{
+
+	struct lib_ring_buffer_backend *bufb = &ctx->buf->backend;
+	struct channel_backend *chanb = &ctx->chan->backend;
+	size_t sbidx, index;
+	size_t offset = ctx->buf_offset;
+	ssize_t pagecpy;
+	struct lib_ring_buffer_backend_pages *rpages;
+	unsigned long sb_bindex, id;
+
+	offset &= chanb->buf_size - 1;
+	sbidx = offset >> chanb->subbuf_size_order;
+	index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
+	pagecpy = min_t(size_t, len, (-offset) & ~PAGE_MASK);
+	id = bufb->buf_wsb[sbidx].id;
+	sb_bindex = subbuffer_id_get_index(config, id);
+	rpages = bufb->array[sb_bindex];
+	CHAN_WARN_ON(ctx->chan,
+		     config->mode == RING_BUFFER_OVERWRITE
+		     && subbuffer_id_is_noref(config, id));
+	if (likely(pagecpy == len))
+		lib_ring_buffer_do_memset(rpages->p[index].virt
+					  + (offset & ~PAGE_MASK),
+					  c, len);
+	else
+		_lib_ring_buffer_memset(bufb, offset, c, len, 0);
+	ctx->buf_offset += len;
+}
+
+/**
+ * lib_ring_buffer_copy_from_user - write userspace data to a buffer backend
+ * @config : ring buffer instance configuration
+ * @ctx: ring buffer context. (input arguments only)
+ * @src : userspace source pointer to copy from
+ * @len : length of data to copy
+ *
+ * This function copies "len" bytes of data from a userspace pointer to a
+ * buffer backend, at the current context offset. This is more or less a buffer
+ * backend-specific memcpy() operation. Calls the slow path
+ * (_ring_buffer_write_from_user) if copy is crossing a page boundary.
+ */
+static inline
+void lib_ring_buffer_copy_from_user(const struct lib_ring_buffer_config *config,
+				    struct lib_ring_buffer_ctx *ctx,
+				    const void __user *src, size_t len)
+{
+	struct lib_ring_buffer_backend *bufb = &ctx->buf->backend;
+	struct channel_backend *chanb = &ctx->chan->backend;
+	size_t sbidx, index;
+	size_t offset = ctx->buf_offset;
+	ssize_t pagecpy;
+	struct lib_ring_buffer_backend_pages *rpages;
+	unsigned long sb_bindex, id;
+	unsigned long ret;
+
+	offset &= chanb->buf_size - 1;
+	sbidx = offset >> chanb->subbuf_size_order;
+	index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
+	pagecpy = min_t(size_t, len, (-offset) & ~PAGE_MASK);
+	id = bufb->buf_wsb[sbidx].id;
+	sb_bindex = subbuffer_id_get_index(config, id);
+	rpages = bufb->array[sb_bindex];
+	CHAN_WARN_ON(ctx->chan,
+		     config->mode == RING_BUFFER_OVERWRITE
+		     && subbuffer_id_is_noref(config, id));
+
+	if (unlikely(!access_ok(VERIFY_READ, src, len)))
+		goto fill_buffer;
+
+	if (likely(pagecpy == len)) {
+		ret = lib_ring_buffer_do_copy_from_user(
+			rpages->p[index].virt + (offset & ~PAGE_MASK),
+			src, len);
+		if (unlikely(ret > 0)) {
+			len -= (pagecpy - ret);
+			offset += (pagecpy - ret);
+			goto fill_buffer;
+		}
+	} else {
+		_lib_ring_buffer_copy_from_user(bufb, offset, src, len, 0);
+	}
+	ctx->buf_offset += len;
+
+	return;
+
+fill_buffer:
+	/*
+	 * In the error path we call the slow path version to avoid
+	 * the pollution of static inline code.
+	 */
+	_lib_ring_buffer_memset(bufb, offset, 0, len, 0);
+}
+
+/*
+ * This accessor counts the number of unread records in a buffer.
+ * It only provides a consistent value if no reads not writes are performed
+ * concurrently.
+ */
+static inline
+unsigned long lib_ring_buffer_get_records_unread(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	struct lib_ring_buffer_backend *bufb = &buf->backend;
+	struct lib_ring_buffer_backend_pages *pages;
+	unsigned long records_unread = 0, sb_bindex, id;
+	unsigned int i;
+
+	for (i = 0; i < bufb->chan->backend.num_subbuf; i++) {
+		id = bufb->buf_wsb[i].id;
+		sb_bindex = subbuffer_id_get_index(config, id);
+		pages = bufb->array[sb_bindex];
+		records_unread += v_read(config, &pages->records_unread);
+	}
+	if (config->mode == RING_BUFFER_OVERWRITE) {
+		id = bufb->buf_rsb.id;
+		sb_bindex = subbuffer_id_get_index(config, id);
+		pages = bufb->array[sb_bindex];
+		records_unread += v_read(config, &pages->records_unread);
+	}
+	return records_unread;
+}
+
+ssize_t lib_ring_buffer_file_splice_read(struct file *in, loff_t *ppos,
+					 struct pipe_inode_info *pipe,
+					 size_t len, unsigned int flags);
+loff_t lib_ring_buffer_no_llseek(struct file *file, loff_t offset, int origin);
+
+#endif /* _LINUX_RING_BUFFER_BACKEND_H */

drivers/staging/lttng/lib/ringbuffer/backend_internal.h

+#ifndef _LINUX_RING_BUFFER_BACKEND_INTERNAL_H
+#define _LINUX_RING_BUFFER_BACKEND_INTERNAL_H
+
+/*
+ * linux/ringbuffer/backend_internal.h
+ *
+ * Copyright (C) 2008-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring buffer backend (internal helpers).
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include "../../wrapper/ringbuffer/config.h"
+#include "../../wrapper/ringbuffer/backend_types.h"
+#include "../../wrapper/ringbuffer/frontend_types.h"
+#include <linux/string.h>
+#include <linux/uaccess.h>
+
+/* Ring buffer backend API presented to the frontend */
+
+/* Ring buffer and channel backend create/free */
+
+int lib_ring_buffer_backend_create(struct lib_ring_buffer_backend *bufb,
+				   struct channel_backend *chan, int cpu);
+void channel_backend_unregister_notifiers(struct channel_backend *chanb);
+void lib_ring_buffer_backend_free(struct lib_ring_buffer_backend *bufb);
+int channel_backend_init(struct channel_backend *chanb,
+			 const char *name,
+			 const struct lib_ring_buffer_config *config,
+			 void *priv, size_t subbuf_size,
+			 size_t num_subbuf);
+void channel_backend_free(struct channel_backend *chanb);
+
+void lib_ring_buffer_backend_reset(struct lib_ring_buffer_backend *bufb);
+void channel_backend_reset(struct channel_backend *chanb);
+
+int lib_ring_buffer_backend_init(void);
+void lib_ring_buffer_backend_exit(void);
+
+extern void _lib_ring_buffer_write(struct lib_ring_buffer_backend *bufb,
+				   size_t offset, const void *src, size_t len,
+				   ssize_t pagecpy);
+extern void _lib_ring_buffer_memset(struct lib_ring_buffer_backend *bufb,
+				    size_t offset, int c, size_t len,
+				    ssize_t pagecpy);
+extern void _lib_ring_buffer_copy_from_user(struct lib_ring_buffer_backend *bufb,
+					    size_t offset, const void *src,
+					    size_t len, ssize_t pagecpy);
+
+/*
+ * Subbuffer ID bits for overwrite mode. Need to fit within a single word to be
+ * exchanged atomically.
+ *
+ * Top half word, except lowest bit, belongs to "offset", which is used to keep
+ * to count the produced buffers.  For overwrite mode, this provides the
+ * consumer with the capacity to read subbuffers in order, handling the
+ * situation where producers would write up to 2^15 buffers (or 2^31 for 64-bit
+ * systems) concurrently with a single execution of get_subbuf (between offset
+ * sampling and subbuffer ID exchange).
+ */
+
+#define HALF_ULONG_BITS		(BITS_PER_LONG >> 1)
+
+#define SB_ID_OFFSET_SHIFT	(HALF_ULONG_BITS + 1)
+#define SB_ID_OFFSET_COUNT	(1UL << SB_ID_OFFSET_SHIFT)
+#define SB_ID_OFFSET_MASK	(~(SB_ID_OFFSET_COUNT - 1))
+/*
+ * Lowest bit of top word half belongs to noref. Used only for overwrite mode.
+ */
+#define SB_ID_NOREF_SHIFT	(SB_ID_OFFSET_SHIFT - 1)
+#define SB_ID_NOREF_COUNT	(1UL << SB_ID_NOREF_SHIFT)
+#define SB_ID_NOREF_MASK	SB_ID_NOREF_COUNT
+/*
+ * In overwrite mode: lowest half of word is used for index.
+ * Limit of 2^16 subbuffers per buffer on 32-bit, 2^32 on 64-bit.
+ * In producer-consumer mode: whole word used for index.
+ */
+#define SB_ID_INDEX_SHIFT	0
+#define SB_ID_INDEX_COUNT	(1UL << SB_ID_INDEX_SHIFT)
+#define SB_ID_INDEX_MASK	(SB_ID_NOREF_COUNT - 1)
+
+/*
+ * Construct the subbuffer id from offset, index and noref. Use only the index
+ * for producer-consumer mode (offset and noref are only used in overwrite
+ * mode).
+ */
+static inline
+unsigned long subbuffer_id(const struct lib_ring_buffer_config *config,
+			   unsigned long offset, unsigned long noref,
+			   unsigned long index)
+{
+	if (config->mode == RING_BUFFER_OVERWRITE)
+		return (offset << SB_ID_OFFSET_SHIFT)
+		       | (noref << SB_ID_NOREF_SHIFT)
+		       | index;
+	else
+		return index;
+}
+
+/*
+ * Compare offset with the offset contained within id. Return 1 if the offset
+ * bits are identical, else 0.
+ */
+static inline
+int subbuffer_id_compare_offset(const struct lib_ring_buffer_config *config,
+				unsigned long id, unsigned long offset)
+{
+	return (id & SB_ID_OFFSET_MASK) == (offset << SB_ID_OFFSET_SHIFT);
+}
+
+static inline
+unsigned long subbuffer_id_get_index(const struct lib_ring_buffer_config *config,
+				     unsigned long id)
+{
+	if (config->mode == RING_BUFFER_OVERWRITE)
+		return id & SB_ID_INDEX_MASK;
+	else
+		return id;
+}
+
+static inline
+unsigned long subbuffer_id_is_noref(const struct lib_ring_buffer_config *config,
+				    unsigned long id)
+{
+	if (config->mode == RING_BUFFER_OVERWRITE)
+		return !!(id & SB_ID_NOREF_MASK);
+	else
+		return 1;
+}
+
+/*
+ * Only used by reader on subbuffer ID it has exclusive access to. No volatile
+ * needed.
+ */
+static inline
+void subbuffer_id_set_noref(const struct lib_ring_buffer_config *config,
+			    unsigned long *id)
+{
+	if (config->mode == RING_BUFFER_OVERWRITE)
+		*id |= SB_ID_NOREF_MASK;
+}
+
+static inline
+void subbuffer_id_set_noref_offset(const struct lib_ring_buffer_config *config,
+				   unsigned long *id, unsigned long offset)
+{
+	unsigned long tmp;
+
+	if (config->mode == RING_BUFFER_OVERWRITE) {
+		tmp = *id;
+		tmp &= ~SB_ID_OFFSET_MASK;
+		tmp |= offset << SB_ID_OFFSET_SHIFT;
+		tmp |= SB_ID_NOREF_MASK;
+		/* Volatile store, read concurrently by readers. */
+		ACCESS_ONCE(*id) = tmp;
+	}
+}
+
+/* No volatile access, since already used locally */
+static inline
+void subbuffer_id_clear_noref(const struct lib_ring_buffer_config *config,
+			      unsigned long *id)
+{
+	if (config->mode == RING_BUFFER_OVERWRITE)
+		*id &= ~SB_ID_NOREF_MASK;
+}
+
+/*
+ * For overwrite mode, cap the number of subbuffers per buffer to:
+ * 2^16 on 32-bit architectures
+ * 2^32 on 64-bit architectures
+ * This is required to fit in the index part of the ID. Return 0 on success,
+ * -EPERM on failure.
+ */
+static inline
+int subbuffer_id_check_index(const struct lib_ring_buffer_config *config,
+			     unsigned long num_subbuf)
+{
+	if (config->mode == RING_BUFFER_OVERWRITE)
+		return (num_subbuf > (1UL << HALF_ULONG_BITS)) ? -EPERM : 0;
+	else
+		return 0;
+}
+
+static inline
+void subbuffer_count_record(const struct lib_ring_buffer_config *config,
+			    struct lib_ring_buffer_backend *bufb,
+			    unsigned long idx)
+{
+	unsigned long sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
+	v_inc(config, &bufb->array[sb_bindex]->records_commit);
+}
+
+/*
+ * Reader has exclusive subbuffer access for record consumption. No need to
+ * perform the decrement atomically.
+ */
+static inline
+void subbuffer_consume_record(const struct lib_ring_buffer_config *config,
+			      struct lib_ring_buffer_backend *bufb)
+{
+	unsigned long sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
+	CHAN_WARN_ON(bufb->chan,
+		     !v_read(config, &bufb->array[sb_bindex]->records_unread));
+	/* Non-atomic decrement protected by exclusive subbuffer access */
+	_v_dec(config, &bufb->array[sb_bindex]->records_unread);
+	v_inc(config, &bufb->records_read);
+}
+
+static inline
+unsigned long subbuffer_get_records_count(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer_backend *bufb,
+				unsigned long idx)
+{
+	unsigned long sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
+	return v_read(config, &bufb->array[sb_bindex]->records_commit);
+}
+
+/*
+ * Must be executed at subbuffer delivery when the writer has _exclusive_
+ * subbuffer access. See ring_buffer_check_deliver() for details.
+ * ring_buffer_get_records_count() must be called to get the records count
+ * before this function, because it resets the records_commit count.
+ */
+static inline
+unsigned long subbuffer_count_records_overrun(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer_backend *bufb,
+				unsigned long idx)
+{
+	struct lib_ring_buffer_backend_pages *pages;
+	unsigned long overruns, sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
+	pages = bufb->array[sb_bindex];
+	overruns = v_read(config, &pages->records_unread);
+	v_set(config, &pages->records_unread,
+	      v_read(config, &pages->records_commit));
+	v_set(config, &pages->records_commit, 0);
+
+	return overruns;
+}
+
+static inline
+void subbuffer_set_data_size(const struct lib_ring_buffer_config *config,
+			     struct lib_ring_buffer_backend *bufb,
+			     unsigned long idx,
+			     unsigned long data_size)
+{
+	struct lib_ring_buffer_backend_pages *pages;
+	unsigned long sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
+	pages = bufb->array[sb_bindex];
+	pages->data_size = data_size;
+}
+
+static inline
+unsigned long subbuffer_get_read_data_size(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer_backend *bufb)
+{
+	struct lib_ring_buffer_backend_pages *pages;
+	unsigned long sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_rsb.id);
+	pages = bufb->array[sb_bindex];
+	return pages->data_size;
+}
+
+static inline
+unsigned long subbuffer_get_data_size(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer_backend *bufb,
+				unsigned long idx)
+{
+	struct lib_ring_buffer_backend_pages *pages;
+	unsigned long sb_bindex;
+
+	sb_bindex = subbuffer_id_get_index(config, bufb->buf_wsb[idx].id);
+	pages = bufb->array[sb_bindex];
+	return pages->data_size;
+}
+
+/**
+ * lib_ring_buffer_clear_noref - Clear the noref subbuffer flag, called by
+ *                               writer.
+ */
+static inline
+void lib_ring_buffer_clear_noref(const struct lib_ring_buffer_config *config,
+				 struct lib_ring_buffer_backend *bufb,
+				 unsigned long idx)
+{
+	unsigned long id, new_id;
+
+	if (config->mode != RING_BUFFER_OVERWRITE)
+		return;
+
+	/*
+	 * Performing a volatile access to read the sb_pages, because we want to
+	 * read a coherent version of the pointer and the associated noref flag.
+	 */
+	id = ACCESS_ONCE(bufb->buf_wsb[idx].id);
+	for (;;) {
+		/* This check is called on the fast path for each record. */
+		if (likely(!subbuffer_id_is_noref(config, id))) {
+			/*
+			 * Store after load dependency ordering the writes to
+			 * the subbuffer after load and test of the noref flag
+			 * matches the memory barrier implied by the cmpxchg()
+			 * in update_read_sb_index().
+			 */
+			return;	/* Already writing to this buffer */
+		}
+		new_id = id;
+		subbuffer_id_clear_noref(config, &new_id);
+		new_id = cmpxchg(&bufb->buf_wsb[idx].id, id, new_id);
+		if (likely(new_id == id))
+			break;
+		id = new_id;
+	}
+}
+
+/**
+ * lib_ring_buffer_set_noref_offset - Set the noref subbuffer flag and offset,
+ *                                    called by writer.
+ */
+static inline
+void lib_ring_buffer_set_noref_offset(const struct lib_ring_buffer_config *config,
+				      struct lib_ring_buffer_backend *bufb,
+				      unsigned long idx, unsigned long offset)
+{
+	if (config->mode != RING_BUFFER_OVERWRITE)
+		return;
+
+	/*
+	 * Because ring_buffer_set_noref() is only called by a single thread
+	 * (the one which updated the cc_sb value), there are no concurrent
+	 * updates to take care of: other writers have not updated cc_sb, so
+	 * they cannot set the noref flag, and concurrent readers cannot modify
+	 * the pointer because the noref flag is not set yet.
+	 * The smp_wmb() in ring_buffer_commit() takes care of ordering writes
+	 * to the subbuffer before this set noref operation.
+	 * subbuffer_set_noref() uses a volatile store to deal with concurrent
+	 * readers of the noref flag.
+	 */
+	CHAN_WARN_ON(bufb->chan,
+		     subbuffer_id_is_noref(config, bufb->buf_wsb[idx].id));
+	/*
+	 * Memory barrier that ensures counter stores are ordered before set
+	 * noref and offset.
+	 */
+	smp_mb();
+	subbuffer_id_set_noref_offset(config, &bufb->buf_wsb[idx].id, offset);
+}
+
+/**
+ * update_read_sb_index - Read-side subbuffer index update.
+ */
+static inline
+int update_read_sb_index(const struct lib_ring_buffer_config *config,
+			 struct lib_ring_buffer_backend *bufb,
+			 struct channel_backend *chanb,
+			 unsigned long consumed_idx,
+			 unsigned long consumed_count)
+{
+	unsigned long old_id, new_id;
+
+	if (config->mode == RING_BUFFER_OVERWRITE) {
+		/*
+		 * Exchange the target writer subbuffer with our own unused
+		 * subbuffer. No need to use ACCESS_ONCE() here to read the
+		 * old_wpage, because the value read will be confirmed by the
+		 * following cmpxchg().
+		 */
+		old_id = bufb->buf_wsb[consumed_idx].id;
+		if (unlikely(!subbuffer_id_is_noref(config, old_id)))
+			return -EAGAIN;
+		/*
+		 * Make sure the offset count we are expecting matches the one
+		 * indicated by the writer.
+		 */
+		if (unlikely(!subbuffer_id_compare_offset(config, old_id,
+							  consumed_count)))
+			return -EAGAIN;
+		CHAN_WARN_ON(bufb->chan,
+			     !subbuffer_id_is_noref(config, bufb->buf_rsb.id));
+		subbuffer_id_set_noref_offset(config, &bufb->buf_rsb.id,
+					      consumed_count);
+		new_id = cmpxchg(&bufb->buf_wsb[consumed_idx].id, old_id,
+				 bufb->buf_rsb.id);
+		if (unlikely(old_id != new_id))
+			return -EAGAIN;
+		bufb->buf_rsb.id = new_id;
+	} else {
+		/* No page exchange, use the writer page directly */
+		bufb->buf_rsb.id = bufb->buf_wsb[consumed_idx].id;
+	}
+	return 0;
+}
+
+/*
+ * Use the architecture-specific memcpy implementation for constant-sized
+ * inputs, but rely on an inline memcpy for length statically unknown.
+ * The function call to memcpy is just way too expensive for a fast path.
+ */
+#define lib_ring_buffer_do_copy(config, dest, src, len)		\
+do {								\
+	size_t __len = (len);					\
+	if (__builtin_constant_p(len))				\
+		memcpy(dest, src, __len);			\
+	else							\
+		inline_memcpy(dest, src, __len);		\
+} while (0)
+
+/*
+ * We use __copy_from_user to copy userspace data since we already
+ * did the access_ok for the whole range.
+ */
+static inline
+unsigned long lib_ring_buffer_do_copy_from_user(void *dest,
+						const void __user *src,
+						unsigned long len)
+{
+	return __copy_from_user(dest, src, len);
+}
+
+/*
+ * write len bytes to dest with c
+ */
+static inline
+void lib_ring_buffer_do_memset(char *dest, int c,
+	unsigned long len)
+{
+	unsigned long i;
+
+	for (i = 0; i < len; i++)
+		dest[i] = c;
+}
+
+#endif /* _LINUX_RING_BUFFER_BACKEND_INTERNAL_H */

drivers/staging/lttng/lib/ringbuffer/backend_types.h

+#ifndef _LINUX_RING_BUFFER_BACKEND_TYPES_H
+#define _LINUX_RING_BUFFER_BACKEND_TYPES_H
+
+/*
+ * linux/ringbuffer/backend_types.h
+ *
+ * Copyright (C) 2008-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring buffer backend (types).
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include <linux/cpumask.h>
+#include <linux/types.h>
+
+struct lib_ring_buffer_backend_page {
+	void *virt;			/* page virtual address (cached) */
+	struct page *page;		/* pointer to page structure */
+};
+
+struct lib_ring_buffer_backend_pages {
+	unsigned long mmap_offset;	/* offset of the subbuffer in mmap */
+	union v_atomic records_commit;	/* current records committed count */
+	union v_atomic records_unread;	/* records to read */
+	unsigned long data_size;	/* Amount of data to read from subbuf */
+	struct lib_ring_buffer_backend_page p[];
+};
+
+struct lib_ring_buffer_backend_subbuffer {
+	/* Identifier for subbuf backend pages. Exchanged atomically. */
+	unsigned long id;		/* backend subbuffer identifier */
+};
+
+/*
+ * Forward declaration of frontend-specific channel and ring_buffer.
+ */
+struct channel;
+struct lib_ring_buffer;
+
+struct lib_ring_buffer_backend {
+	/* Array of ring_buffer_backend_subbuffer for writer */
+	struct lib_ring_buffer_backend_subbuffer *buf_wsb;
+	/* ring_buffer_backend_subbuffer for reader */
+	struct lib_ring_buffer_backend_subbuffer buf_rsb;
+	/*
+	 * Pointer array of backend pages, for whole buffer.
+	 * Indexed by ring_buffer_backend_subbuffer identifier (id) index.
+	 */
+	struct lib_ring_buffer_backend_pages **array;
+	unsigned int num_pages_per_subbuf;
+
+	struct channel *chan;		/* Associated channel */
+	int cpu;			/* This buffer's cpu. -1 if global. */
+	union v_atomic records_read;	/* Number of records read */
+	unsigned int allocated:1;	/* Bool: is buffer allocated ? */
+};
+
+struct channel_backend {
+	unsigned long buf_size;		/* Size of the buffer */
+	unsigned long subbuf_size;	/* Sub-buffer size */
+	unsigned int subbuf_size_order;	/* Order of sub-buffer size */
+	unsigned int num_subbuf_order;	/*
+					 * Order of number of sub-buffers/buffer
+					 * for writer.
+					 */
+	unsigned int buf_size_order;	/* Order of buffer size */
+	int extra_reader_sb:1;		/* Bool: has extra reader subbuffer */
+	struct lib_ring_buffer *buf;	/* Channel per-cpu buffers */
+
+	unsigned long num_subbuf;	/* Number of sub-buffers for writer */
+	u64 start_tsc;			/* Channel creation TSC value */
+	void *priv;			/* Client-specific information */
+	struct notifier_block cpu_hp_notifier;	 /* CPU hotplug notifier */
+	const struct lib_ring_buffer_config *config; /* Ring buffer configuration */
+	cpumask_var_t cpumask;		/* Allocated per-cpu buffers cpumask */
+	char name[NAME_MAX];		/* Channel name */
+};
+
+#endif /* _LINUX_RING_BUFFER_BACKEND_TYPES_H */

drivers/staging/lttng/lib/ringbuffer/config.h

+#ifndef _LINUX_RING_BUFFER_CONFIG_H
+#define _LINUX_RING_BUFFER_CONFIG_H
+
+/*
+ * linux/ringbuffer/config.h
+ *
+ * Copyright (C) 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring buffer configuration header. Note: after declaring the standard inline
+ * functions, clients should also include linux/ringbuffer/api.h.
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include <linux/types.h>
+#include <linux/percpu.h>
+#include "../align.h"
+
+struct lib_ring_buffer;
+struct channel;
+struct lib_ring_buffer_config;
+struct lib_ring_buffer_ctx;
+
+/*
+ * Ring buffer client callbacks. Only used by slow path, never on fast path.
+ * For the fast path, record_header_size(), ring_buffer_clock_read() should be
+ * provided as inline functions too.  These may simply return 0 if not used by
+ * the client.
+ */
+struct lib_ring_buffer_client_cb {
+	/* Mandatory callbacks */
+
+	/* A static inline version is also required for fast path */
+	u64 (*ring_buffer_clock_read) (struct channel *chan);
+	size_t (*record_header_size) (const struct lib_ring_buffer_config *config,
+				      struct channel *chan, size_t offset,
+				      size_t *pre_header_padding,
+				      struct lib_ring_buffer_ctx *ctx);
+
+	/* Slow path only, at subbuffer switch */
+	size_t (*subbuffer_header_size) (void);
+	void (*buffer_begin) (struct lib_ring_buffer *buf, u64 tsc,
+			      unsigned int subbuf_idx);
+	void (*buffer_end) (struct lib_ring_buffer *buf, u64 tsc,
+			    unsigned int subbuf_idx, unsigned long data_size);
+
+	/* Optional callbacks (can be set to NULL) */
+
+	/* Called at buffer creation/finalize */
+	int (*buffer_create) (struct lib_ring_buffer *buf, void *priv,
+			      int cpu, const char *name);
+	/*
+	 * Clients should guarantee that no new reader handle can be opened
+	 * after finalize.
+	 */
+	void (*buffer_finalize) (struct lib_ring_buffer *buf, void *priv, int cpu);
+
+	/*
+	 * Extract header length, payload length and timestamp from event
+	 * record. Used by buffer iterators. Timestamp is only used by channel
+	 * iterator.
+	 */
+	void (*record_get) (const struct lib_ring_buffer_config *config,
+			    struct channel *chan, struct lib_ring_buffer *buf,
+			    size_t offset, size_t *header_len,
+			    size_t *payload_len, u64 *timestamp);
+};
+
+/*
+ * Ring buffer instance configuration.
+ *
+ * Declare as "static const" within the client object to ensure the inline fast
+ * paths can be optimized.
+ *
+ * alloc/sync pairs:
+ *
+ * RING_BUFFER_ALLOC_PER_CPU and RING_BUFFER_SYNC_PER_CPU :
+ *   Per-cpu buffers with per-cpu synchronization. Tracing must be performed
+ *   with preemption disabled (lib_ring_buffer_get_cpu() and
+ *   lib_ring_buffer_put_cpu()).
+ *
+ * RING_BUFFER_ALLOC_PER_CPU and RING_BUFFER_SYNC_GLOBAL :
+ *   Per-cpu buffer with global synchronization. Tracing can be performed with
+ *   preemption enabled, statistically stays on the local buffers.
+ *
+ * RING_BUFFER_ALLOC_GLOBAL and RING_BUFFER_SYNC_PER_CPU :
+ *   Should only be used for buffers belonging to a single thread or protected
+ *   by mutual exclusion by the client. Note that periodical sub-buffer switch
+ *   should be disabled in this kind of configuration.
+ *
+ * RING_BUFFER_ALLOC_GLOBAL and RING_BUFFER_SYNC_GLOBAL :
+ *   Global shared buffer with global synchronization.
+ *
+ * wakeup:
+ *
+ * RING_BUFFER_WAKEUP_BY_TIMER uses per-cpu deferrable timers to poll the
+ * buffers and wake up readers if data is ready. Mainly useful for tracers which
+ * don't want to call into the wakeup code on the tracing path. Use in
+ * combination with "read_timer_interval" channel_create() argument.
+ *
+ * RING_BUFFER_WAKEUP_BY_WRITER directly wakes up readers when a subbuffer is
+ * ready to read. Lower latencies before the reader is woken up. Mainly suitable
+ * for drivers.
+ *
+ * RING_BUFFER_WAKEUP_NONE does not perform any wakeup whatsoever. The client
+ * has the responsibility to perform wakeups.
+ */
+struct lib_ring_buffer_config {
+	enum {
+		RING_BUFFER_ALLOC_PER_CPU,
+		RING_BUFFER_ALLOC_GLOBAL,
+	} alloc;
+	enum {
+		RING_BUFFER_SYNC_PER_CPU,	/* Wait-free */
+		RING_BUFFER_SYNC_GLOBAL,	/* Lock-free */
+	} sync;
+	enum {
+		RING_BUFFER_OVERWRITE,		/* Overwrite when buffer full */
+		RING_BUFFER_DISCARD,		/* Discard when buffer full */
+	} mode;
+	enum {
+		RING_BUFFER_SPLICE,
+		RING_BUFFER_MMAP,
+		RING_BUFFER_READ,		/* TODO */
+		RING_BUFFER_ITERATOR,
+		RING_BUFFER_NONE,
+	} output;
+	enum {
+		RING_BUFFER_PAGE,
+		RING_BUFFER_VMAP,		/* TODO */
+		RING_BUFFER_STATIC,		/* TODO */
+	} backend;
+	enum {
+		RING_BUFFER_NO_OOPS_CONSISTENCY,
+		RING_BUFFER_OOPS_CONSISTENCY,
+	} oops;
+	enum {
+		RING_BUFFER_IPI_BARRIER,
+		RING_BUFFER_NO_IPI_BARRIER,
+	} ipi;
+	enum {
+		RING_BUFFER_WAKEUP_BY_TIMER,	/* wake up performed by timer */
+		RING_BUFFER_WAKEUP_BY_WRITER,	/*
+						 * writer wakes up reader,
+						 * not lock-free
+						 * (takes spinlock).
+						 */
+	} wakeup;
+	/*
+	 * tsc_bits: timestamp bits saved at each record.
+	 *   0 and 64 disable the timestamp compression scheme.
+	 */
+	unsigned int tsc_bits;
+	struct lib_ring_buffer_client_cb cb;
+};
+
+/*
+ * ring buffer context
+ *
+ * Context passed to lib_ring_buffer_reserve(), lib_ring_buffer_commit(),
+ * lib_ring_buffer_try_discard_reserve(), lib_ring_buffer_align_ctx() and
+ * lib_ring_buffer_write().
+ */
+struct lib_ring_buffer_ctx {
+	/* input received by lib_ring_buffer_reserve(), saved here. */
+	struct channel *chan;		/* channel */
+	void *priv;			/* client private data */
+	size_t data_size;		/* size of payload */
+	int largest_align;		/*
+					 * alignment of the largest element
+					 * in the payload
+					 */
+	int cpu;			/* processor id */
+
+	/* output from lib_ring_buffer_reserve() */
+	struct lib_ring_buffer *buf;	/*
+					 * buffer corresponding to processor id
+					 * for this channel
+					 */
+	size_t slot_size;		/* size of the reserved slot */
+	unsigned long buf_offset;	/* offset following the record header */
+	unsigned long pre_offset;	/*
+					 * Initial offset position _before_
+					 * the record is written. Positioned
+					 * prior to record header alignment
+					 * padding.
+					 */
+	u64 tsc;			/* time-stamp counter value */
+	unsigned int rflags;		/* reservation flags */
+};
+
+/**
+ * lib_ring_buffer_ctx_init - initialize ring buffer context
+ * @ctx: ring buffer context to initialize
+ * @chan: channel
+ * @priv: client private data
+ * @data_size: size of record data payload
+ * @largest_align: largest alignment within data payload types
+ * @cpu: processor id
+ */
+static inline
+void lib_ring_buffer_ctx_init(struct lib_ring_buffer_ctx *ctx,
+			      struct channel *chan, void *priv,
+			      size_t data_size, int largest_align,
+			      int cpu)
+{
+	ctx->chan = chan;
+	ctx->priv = priv;
+	ctx->data_size = data_size;
+	ctx->largest_align = largest_align;
+	ctx->cpu = cpu;
+	ctx->rflags = 0;
+}
+
+/*
+ * Reservation flags.
+ *
+ * RING_BUFFER_RFLAG_FULL_TSC
+ *
+ * This flag is passed to record_header_size() and to the primitive used to
+ * write the record header. It indicates that the full 64-bit time value is
+ * needed in the record header. If this flag is not set, the record header needs
+ * only to contain "tsc_bits" bit of time value.
+ *
+ * Reservation flags can be added by the client, starting from
+ * "(RING_BUFFER_FLAGS_END << 0)". It can be used to pass information from
+ * record_header_size() to lib_ring_buffer_write_record_header().
+ */
+#define	RING_BUFFER_RFLAG_FULL_TSC		(1U << 0)
+#define RING_BUFFER_RFLAG_END			(1U << 1)
+
+/*
+ * We need to define RING_BUFFER_ALIGN_ATTR so it is known early at
+ * compile-time. We have to duplicate the "config->align" information and the
+ * definition here because config->align is used both in the slow and fast
+ * paths, but RING_BUFFER_ALIGN_ATTR is only available for the client code.
+ */
+#ifdef RING_BUFFER_ALIGN
+
+# define RING_BUFFER_ALIGN_ATTR		/* Default arch alignment */
+
+/*
+ * Calculate the offset needed to align the type.
+ * size_of_type must be non-zero.
+ */
+static inline
+unsigned int lib_ring_buffer_align(size_t align_drift, size_t size_of_type)
+{
+	return offset_align(align_drift, size_of_type);
+}
+
+#else
+
+# define RING_BUFFER_ALIGN_ATTR __attribute__((packed))
+
+/*
+ * Calculate the offset needed to align the type.
+ * size_of_type must be non-zero.
+ */
+static inline
+unsigned int lib_ring_buffer_align(size_t align_drift, size_t size_of_type)
+{
+	return 0;
+}
+
+#endif
+
+/**
+ * lib_ring_buffer_align_ctx - Align context offset on "alignment"
+ * @ctx: ring buffer context.
+ */
+static inline
+void lib_ring_buffer_align_ctx(struct lib_ring_buffer_ctx *ctx,
+			   size_t alignment)
+{
+	ctx->buf_offset += lib_ring_buffer_align(ctx->buf_offset,
+						 alignment);
+}
+
+/*
+ * lib_ring_buffer_check_config() returns 0 on success.
+ * Used internally to check for valid configurations at channel creation.
+ */
+static inline
+int lib_ring_buffer_check_config(const struct lib_ring_buffer_config *config,
+			     unsigned int switch_timer_interval,
+			     unsigned int read_timer_interval)
+{
+	if (config->alloc == RING_BUFFER_ALLOC_GLOBAL
+	    && config->sync == RING_BUFFER_SYNC_PER_CPU
+	    && switch_timer_interval)
+		return -EINVAL;
+	return 0;
+}
+
+#include "../../wrapper/ringbuffer/vatomic.h"
+
+#endif /* _LINUX_RING_BUFFER_CONFIG_H */

drivers/staging/lttng/lib/ringbuffer/frontend.h

+#ifndef _LINUX_RING_BUFFER_FRONTEND_H
+#define _LINUX_RING_BUFFER_FRONTEND_H
+
+/*
+ * linux/ringbuffer/frontend.h
+ *
+ * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring Buffer Library Synchronization Header (API).
+ *
+ * Author:
+ *	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * See ring_buffer_frontend.c for more information on wait-free algorithms.
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include <linux/pipe_fs_i.h>
+#include <linux/rcupdate.h>
+#include <linux/cpumask.h>
+#include <linux/module.h>
+#include <linux/bitops.h>
+#include <linux/splice.h>
+#include <linux/string.h>
+#include <linux/timer.h>
+#include <linux/sched.h>
+#include <linux/cache.h>
+#include <linux/time.h>
+#include <linux/slab.h>
+#include <linux/init.h>
+#include <linux/stat.h>
+#include <linux/cpu.h>
+#include <linux/fs.h>
+
+#include <asm/atomic.h>
+#include <asm/local.h>
+
+/* Internal helpers */
+#include "../../wrapper/ringbuffer/frontend_internal.h"
+
+/* Buffer creation/removal and setup operations */
+
+/*
+ * switch_timer_interval is the time interval (in us) to fill sub-buffers with
+ * padding to let readers get those sub-buffers.  Used for live streaming.
+ *
+ * read_timer_interval is the time interval (in us) to wake up pending readers.
+ *
+ * buf_addr is a pointer the the beginning of the preallocated buffer contiguous
+ * address mapping. It is used only by RING_BUFFER_STATIC configuration. It can
+ * be set to NULL for other backends.
+ */
+
+extern
+struct channel *channel_create(const struct lib_ring_buffer_config *config,
+			       const char *name, void *priv,
+			       void *buf_addr,
+			       size_t subbuf_size, size_t num_subbuf,
+			       unsigned int switch_timer_interval,
+			       unsigned int read_timer_interval);
+
+/*
+ * channel_destroy returns the private data pointer. It finalizes all channel's
+ * buffers, waits for readers to release all references, and destroys the
+ * channel.
+ */
+extern
+void *channel_destroy(struct channel *chan);
+
+
+/* Buffer read operations */
+
+/*
+ * Iteration on channel cpumask needs to issue a read barrier to match the write
+ * barrier in cpu hotplug. It orders the cpumask read before read of per-cpu
+ * buffer data. The per-cpu buffer is never removed by cpu hotplug; teardown is
+ * only performed at channel destruction.
+ */
+#define for_each_channel_cpu(cpu, chan)					\
+	for ((cpu) = -1;						\
+		({ (cpu) = cpumask_next(cpu, (chan)->backend.cpumask);	\
+		   smp_read_barrier_depends(); (cpu) < nr_cpu_ids; });)
+
+extern struct lib_ring_buffer *channel_get_ring_buffer(
+				const struct lib_ring_buffer_config *config,
+				struct channel *chan, int cpu);
+extern int lib_ring_buffer_open_read(struct lib_ring_buffer *buf);
+extern void lib_ring_buffer_release_read(struct lib_ring_buffer *buf);
+
+/*
+ * Read sequence: snapshot, many get_subbuf/put_subbuf, move_consumer.
+ */
+extern int lib_ring_buffer_snapshot(struct lib_ring_buffer *buf,
+				    unsigned long *consumed,
+				    unsigned long *produced);
+extern void lib_ring_buffer_move_consumer(struct lib_ring_buffer *buf,
+					  unsigned long consumed_new);
+
+extern int lib_ring_buffer_get_subbuf(struct lib_ring_buffer *buf,
+				      unsigned long consumed);
+extern void lib_ring_buffer_put_subbuf(struct lib_ring_buffer *buf);
+
+/*
+ * lib_ring_buffer_get_next_subbuf/lib_ring_buffer_put_next_subbuf are helpers
+ * to read sub-buffers sequentially.
+ */
+static inline int lib_ring_buffer_get_next_subbuf(struct lib_ring_buffer *buf)
+{
+	int ret;
+
+	ret = lib_ring_buffer_snapshot(buf, &buf->cons_snapshot,
+				       &buf->prod_snapshot);
+	if (ret)
+		return ret;
+	ret = lib_ring_buffer_get_subbuf(buf, buf->cons_snapshot);
+	return ret;
+}
+
+static inline void lib_ring_buffer_put_next_subbuf(struct lib_ring_buffer *buf)
+{
+	lib_ring_buffer_put_subbuf(buf);
+	lib_ring_buffer_move_consumer(buf, subbuf_align(buf->cons_snapshot,
+						    buf->backend.chan));
+}
+
+extern void channel_reset(struct channel *chan);
+extern void lib_ring_buffer_reset(struct lib_ring_buffer *buf);
+
+static inline
+unsigned long lib_ring_buffer_get_offset(const struct lib_ring_buffer_config *config,
+					 struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->offset);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_consumed(const struct lib_ring_buffer_config *config,
+					   struct lib_ring_buffer *buf)
+{
+	return atomic_long_read(&buf->consumed);
+}
+
+/*
+ * Must call lib_ring_buffer_is_finalized before reading counters (memory
+ * ordering enforced with respect to trace teardown).
+ */
+static inline
+int lib_ring_buffer_is_finalized(const struct lib_ring_buffer_config *config,
+				 struct lib_ring_buffer *buf)
+{
+	int finalized = ACCESS_ONCE(buf->finalized);
+	/*
+	 * Read finalized before counters.
+	 */
+	smp_rmb();
+	return finalized;
+}
+
+static inline
+int lib_ring_buffer_channel_is_finalized(const struct channel *chan)
+{
+	return chan->finalized;
+}
+
+static inline
+int lib_ring_buffer_channel_is_disabled(const struct channel *chan)
+{
+	return atomic_read(&chan->record_disabled);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_read_data_size(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return subbuffer_get_read_data_size(config, &buf->backend);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_records_count(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->records_count);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_records_overrun(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->records_overrun);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_records_lost_full(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->records_lost_full);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_records_lost_wrap(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->records_lost_wrap);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_records_lost_big(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->records_lost_big);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_records_read(
+				const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer *buf)
+{
+	return v_read(config, &buf->backend.records_read);
+}
+
+#endif /* _LINUX_RING_BUFFER_FRONTEND_H */

drivers/staging/lttng/lib/ringbuffer/frontend_api.h

+#ifndef _LINUX_RING_BUFFER_FRONTEND_API_H
+#define _LINUX_RING_BUFFER_FRONTEND_API_H
+
+/*
+ * linux/ringbuffer/frontend_api.h
+ *
+ * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring Buffer Library Synchronization Header (buffer write API).
+ *
+ * Author:
+ *	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * See ring_buffer_frontend.c for more information on wait-free algorithms.
+ * See linux/ringbuffer/frontend.h for channel allocation and read-side API.
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include "../../wrapper/ringbuffer/frontend.h"
+#include <linux/errno.h>
+
+/**
+ * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
+ *
+ * Disables preemption (acts as a RCU read-side critical section) and keeps a
+ * ring buffer nesting count as supplementary safety net to ensure tracer client
+ * code will never trigger an endless recursion. Returns the processor ID on
+ * success, -EPERM on failure (nesting count too high).
+ *
+ * asm volatile and "memory" clobber prevent the compiler from moving
+ * instructions out of the ring buffer nesting count. This is required to ensure
+ * that probe side-effects which can cause recursion (e.g. unforeseen traps,
+ * divisions by 0, ...) are triggered within the incremented nesting count
+ * section.
+ */
+static inline
+int lib_ring_buffer_get_cpu(const struct lib_ring_buffer_config *config)
+{
+	int cpu, nesting;
+
+	rcu_read_lock_sched_notrace();
+	cpu = smp_processor_id();
+	nesting = ++per_cpu(lib_ring_buffer_nesting, cpu);
+	barrier();
+
+	if (unlikely(nesting > 4)) {
+		WARN_ON_ONCE(1);
+		per_cpu(lib_ring_buffer_nesting, cpu)--;
+		rcu_read_unlock_sched_notrace();
+		return -EPERM;
+	} else
+		return cpu;
+}
+
+/**
+ * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
+ */
+static inline
+void lib_ring_buffer_put_cpu(const struct lib_ring_buffer_config *config)
+{
+	barrier();
+	__get_cpu_var(lib_ring_buffer_nesting)--;
+	rcu_read_unlock_sched_notrace();
+}
+
+/*
+ * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
+ * part of the API per se.
+ *
+ * returns 0 if reserve ok, or 1 if the slow path must be taken.
+ */
+static inline
+int lib_ring_buffer_try_reserve(const struct lib_ring_buffer_config *config,
+				struct lib_ring_buffer_ctx *ctx,
+				unsigned long *o_begin, unsigned long *o_end,
+				unsigned long *o_old, size_t *before_hdr_pad)
+{
+	struct channel *chan = ctx->chan;
+	struct lib_ring_buffer *buf = ctx->buf;
+	*o_begin = v_read(config, &buf->offset);
+	*o_old = *o_begin;
+
+	ctx->tsc = lib_ring_buffer_clock_read(chan);
+	if ((int64_t) ctx->tsc == -EIO)
+		return 1;
+
+	/*
+	 * Prefetch cacheline for read because we have to read the previous
+	 * commit counter to increment it and commit seq value to compare it to
+	 * the commit counter.
+	 */
+	prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
+
+	if (last_tsc_overflow(config, buf, ctx->tsc))
+		ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
+
+	if (unlikely(subbuf_offset(*o_begin, chan) == 0))
+		return 1;
+
+	ctx->slot_size = record_header_size(config, chan, *o_begin,
+					    before_hdr_pad, ctx);
+	ctx->slot_size +=
+		lib_ring_buffer_align(*o_begin + ctx->slot_size,
+				      ctx->largest_align) + ctx->data_size;
+	if (unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
+		     > chan->backend.subbuf_size))
+		return 1;
+
+	/*
+	 * Record fits in the current buffer and we are not on a switch
+	 * boundary. It's safe to write.
+	 */
+	*o_end = *o_begin + ctx->slot_size;
+
+	if (unlikely((subbuf_offset(*o_end, chan)) == 0))
+		/*
+		 * The offset_end will fall at the very beginning of the next
+		 * subbuffer.
+		 */
+		return 1;
+
+	return 0;
+}
+
+/**
+ * lib_ring_buffer_reserve - Reserve space in a ring buffer.
+ * @config: ring buffer instance configuration.
+ * @ctx: ring buffer context. (input and output) Must be already initialized.
+ *
+ * Atomic wait-free slot reservation. The reserved space starts at the context
+ * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
+ *
+ * Return :
+ *  0 on success.
+ * -EAGAIN if channel is disabled.
+ * -ENOSPC if event size is too large for packet.
+ * -ENOBUFS if there is currently not enough space in buffer for the event.
+ * -EIO if data cannot be written into the buffer for any other reason.
+ */
+
+static inline
+int lib_ring_buffer_reserve(const struct lib_ring_buffer_config *config,
+			    struct lib_ring_buffer_ctx *ctx)
+{
+	struct channel *chan = ctx->chan;
+	struct lib_ring_buffer *buf;
+	unsigned long o_begin, o_end, o_old;
+	size_t before_hdr_pad = 0;
+
+	if (atomic_read(&chan->record_disabled))
+		return -EAGAIN;
+
+	if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
+		buf = per_cpu_ptr(chan->backend.buf, ctx->cpu);
+	else
+		buf = chan->backend.buf;
+	if (atomic_read(&buf->record_disabled))
+		return -EAGAIN;
+	ctx->buf = buf;
+
+	/*
+	 * Perform retryable operations.
+	 */
+	if (unlikely(lib_ring_buffer_try_reserve(config, ctx, &o_begin,
+						 &o_end, &o_old, &before_hdr_pad)))
+		goto slow_path;
+
+	if (unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
+		     != o_old))
+		goto slow_path;
+
+	/*
+	 * Atomically update last_tsc. This update races against concurrent
+	 * atomic updates, but the race will always cause supplementary full TSC
+	 * record headers, never the opposite (missing a full TSC record header
+	 * when it would be needed).
+	 */
+	save_last_tsc(config, ctx->buf, ctx->tsc);
+
+	/*
+	 * Push the reader if necessary
+	 */
+	lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
+
+	/*
+	 * Clear noref flag for this subbuffer.
+	 */
+	lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
+				subbuf_index(o_end - 1, chan));
+
+	ctx->pre_offset = o_begin;
+	ctx->buf_offset = o_begin + before_hdr_pad;
+	return 0;
+slow_path:
+	return lib_ring_buffer_reserve_slow(ctx);
+}
+
+/**
+ * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
+ * @config: ring buffer instance configuration.
+ * @buf: buffer
+ * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
+ *
+ * This operation is completely reentrant : can be called while tracing is
+ * active with absolutely no lock held.
+ *
+ * Note, however, that as a v_cmpxchg is used for some atomic operations and
+ * requires to be executed locally for per-CPU buffers, this function must be
+ * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
+ * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
+ */
+static inline
+void lib_ring_buffer_switch(const struct lib_ring_buffer_config *config,
+			    struct lib_ring_buffer *buf, enum switch_mode mode)
+{
+	lib_ring_buffer_switch_slow(buf, mode);
+}
+
+/* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
+
+/**
+ * lib_ring_buffer_commit - Commit an record.
+ * @config: ring buffer instance configuration.
+ * @ctx: ring buffer context. (input arguments only)
+ *
+ * Atomic unordered slot commit. Increments the commit count in the
+ * specified sub-buffer, and delivers it if necessary.
+ */
+static inline
+void lib_ring_buffer_commit(const struct lib_ring_buffer_config *config,
+			    const struct lib_ring_buffer_ctx *ctx)
+{
+	struct channel *chan = ctx->chan;
+	struct lib_ring_buffer *buf = ctx->buf;
+	unsigned long offset_end = ctx->buf_offset;
+	unsigned long endidx = subbuf_index(offset_end - 1, chan);
+	unsigned long commit_count;
+
+	/*
+	 * Must count record before incrementing the commit count.
+	 */
+	subbuffer_count_record(config, &buf->backend, endidx);
+
+	/*
+	 * Order all writes to buffer before the commit count update that will
+	 * determine that the subbuffer is full.
+	 */
+	if (config->ipi == RING_BUFFER_IPI_BARRIER) {
+		/*
+		 * Must write slot data before incrementing commit count.  This
+		 * compiler barrier is upgraded into a smp_mb() by the IPI sent
+		 * by get_subbuf().
+		 */
+		barrier();
+	} else
+		smp_wmb();
+
+	v_add(config, ctx->slot_size, &buf->commit_hot[endidx].cc);
+
+	/*
+	 * commit count read can race with concurrent OOO commit count updates.
+	 * This is only needed for lib_ring_buffer_check_deliver (for
+	 * non-polling delivery only) and for
+	 * lib_ring_buffer_write_commit_counter.  The race can only cause the
+	 * counter to be read with the same value more than once, which could
+	 * cause :
+	 * - Multiple delivery for the same sub-buffer (which is handled
+	 *   gracefully by the reader code) if the value is for a full
+	 *   sub-buffer. It's important that we can never miss a sub-buffer
+	 *   delivery. Re-reading the value after the v_add ensures this.
+	 * - Reading a commit_count with a higher value that what was actually
+	 *   added to it for the lib_ring_buffer_write_commit_counter call
+	 *   (again caused by a concurrent committer). It does not matter,
+	 *   because this function is interested in the fact that the commit
+	 *   count reaches back the reserve offset for a specific sub-buffer,
+	 *   which is completely independent of the order.
+	 */
+	commit_count = v_read(config, &buf->commit_hot[endidx].cc);
+
+	lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
+				      commit_count, endidx);
+	/*
+	 * Update used size at each commit. It's needed only for extracting
+	 * ring_buffer buffers from vmcore, after crash.
+	 */
+	lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
+					     ctx->buf_offset, commit_count,
+					 ctx->slot_size);
+}
+
+/**
+ * lib_ring_buffer_try_discard_reserve - Try discarding a record.
+ * @config: ring buffer instance configuration.
+ * @ctx: ring buffer context. (input arguments only)
+ *
+ * Only succeeds if no other record has been written after the record to
+ * discard. If discard fails, the record must be committed to the buffer.
+ *
+ * Returns 0 upon success, -EPERM if the record cannot be discarded.
+ */
+static inline
+int lib_ring_buffer_try_discard_reserve(const struct lib_ring_buffer_config *config,
+					const struct lib_ring_buffer_ctx *ctx)
+{
+	struct lib_ring_buffer *buf = ctx->buf;
+	unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
+
+	/*
+	 * We need to ensure that if the cmpxchg succeeds and discards the
+	 * record, the next record will record a full TSC, because it cannot
+	 * rely on the last_tsc associated with the discarded record to detect
+	 * overflows. The only way to ensure this is to set the last_tsc to 0
+	 * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
+	 * timestamp in the next record.
+	 *
+	 * Note: if discard fails, we must leave the TSC in the record header.
+	 * It is needed to keep track of TSC overflows for the following
+	 * records.
+	 */
+	save_last_tsc(config, buf, 0ULL);
+
+	if (likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
+		   != end_offset))
+		return -EPERM;
+	else
+		return 0;
+}
+
+static inline
+void channel_record_disable(const struct lib_ring_buffer_config *config,
+			    struct channel *chan)
+{
+	atomic_inc(&chan->record_disabled);
+}
+
+static inline
+void channel_record_enable(const struct lib_ring_buffer_config *config,
+			   struct channel *chan)
+{
+	atomic_dec(&chan->record_disabled);
+}
+
+static inline
+void lib_ring_buffer_record_disable(const struct lib_ring_buffer_config *config,
+				    struct lib_ring_buffer *buf)
+{
+	atomic_inc(&buf->record_disabled);
+}
+
+static inline
+void lib_ring_buffer_record_enable(const struct lib_ring_buffer_config *config,
+				   struct lib_ring_buffer *buf)
+{
+	atomic_dec(&buf->record_disabled);
+}
+
+#endif /* _LINUX_RING_BUFFER_FRONTEND_API_H */

drivers/staging/lttng/lib/ringbuffer/frontend_internal.h

+#ifndef _LINUX_RING_BUFFER_FRONTEND_INTERNAL_H
+#define _LINUX_RING_BUFFER_FRONTEND_INTERNAL_H
+
+/*
+ * linux/ringbuffer/frontend_internal.h
+ *
+ * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring Buffer Library Synchronization Header (internal helpers).
+ *
+ * Author:
+ *	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * See ring_buffer_frontend.c for more information on wait-free algorithms.
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include "../../wrapper/ringbuffer/config.h"
+#include "../../wrapper/ringbuffer/backend_types.h"
+#include "../../wrapper/ringbuffer/frontend_types.h"
+#include "../../lib/prio_heap/lttng_prio_heap.h"	/* For per-CPU read-side iterator */
+
+/* Buffer offset macros */
+
+/* buf_trunc mask selects only the buffer number. */
+static inline
+unsigned long buf_trunc(unsigned long offset, struct channel *chan)
+{
+	return offset & ~(chan->backend.buf_size - 1);
+
+}
+
+/* Select the buffer number value (counter). */
+static inline
+unsigned long buf_trunc_val(unsigned long offset, struct channel *chan)
+{
+	return buf_trunc(offset, chan) >> chan->backend.buf_size_order;
+}
+
+/* buf_offset mask selects only the offset within the current buffer. */
+static inline
+unsigned long buf_offset(unsigned long offset, struct channel *chan)
+{
+	return offset & (chan->backend.buf_size - 1);
+}
+
+/* subbuf_offset mask selects the offset within the current subbuffer. */
+static inline
+unsigned long subbuf_offset(unsigned long offset, struct channel *chan)
+{
+	return offset & (chan->backend.subbuf_size - 1);
+}
+
+/* subbuf_trunc mask selects the subbuffer number. */
+static inline
+unsigned long subbuf_trunc(unsigned long offset, struct channel *chan)
+{
+	return offset & ~(chan->backend.subbuf_size - 1);
+}
+
+/* subbuf_align aligns the offset to the next subbuffer. */
+static inline
+unsigned long subbuf_align(unsigned long offset, struct channel *chan)
+{
+	return (offset + chan->backend.subbuf_size)
+	       & ~(chan->backend.subbuf_size - 1);
+}
+
+/* subbuf_index returns the index of the current subbuffer within the buffer. */
+static inline
+unsigned long subbuf_index(unsigned long offset, struct channel *chan)
+{
+	return buf_offset(offset, chan) >> chan->backend.subbuf_size_order;
+}
+
+/*
+ * Last TSC comparison functions. Check if the current TSC overflows tsc_bits
+ * bits from the last TSC read. When overflows are detected, the full 64-bit
+ * timestamp counter should be written in the record header. Reads and writes
+ * last_tsc atomically.
+ */
+
+#if (BITS_PER_LONG == 32)
+static inline
+void save_last_tsc(const struct lib_ring_buffer_config *config,
+		   struct lib_ring_buffer *buf, u64 tsc)
+{
+	if (config->tsc_bits == 0 || config->tsc_bits == 64)
+		return;
+
+	/*
+	 * Ensure the compiler performs this update in a single instruction.
+	 */
+	v_set(config, &buf->last_tsc, (unsigned long)(tsc >> config->tsc_bits));
+}
+
+static inline
+int last_tsc_overflow(const struct lib_ring_buffer_config *config,
+		      struct lib_ring_buffer *buf, u64 tsc)
+{
+	unsigned long tsc_shifted;
+
+	if (config->tsc_bits == 0 || config->tsc_bits == 64)
+		return 0;
+
+	tsc_shifted = (unsigned long)(tsc >> config->tsc_bits);
+	if (unlikely(tsc_shifted
+		     - (unsigned long)v_read(config, &buf->last_tsc)))
+		return 1;
+	else
+		return 0;
+}
+#else
+static inline
+void save_last_tsc(const struct lib_ring_buffer_config *config,
+		   struct lib_ring_buffer *buf, u64 tsc)
+{
+	if (config->tsc_bits == 0 || config->tsc_bits == 64)
+		return;
+
+	v_set(config, &buf->last_tsc, (unsigned long)tsc);
+}
+
+static inline
+int last_tsc_overflow(const struct lib_ring_buffer_config *config,
+		      struct lib_ring_buffer *buf, u64 tsc)
+{
+	if (config->tsc_bits == 0 || config->tsc_bits == 64)
+		return 0;
+
+	if (unlikely((tsc - v_read(config, &buf->last_tsc))
+		     >> config->tsc_bits))
+		return 1;
+	else
+		return 0;
+}
+#endif
+
+extern
+int lib_ring_buffer_reserve_slow(struct lib_ring_buffer_ctx *ctx);
+
+extern
+void lib_ring_buffer_switch_slow(struct lib_ring_buffer *buf,
+				 enum switch_mode mode);
+
+/* Buffer write helpers */
+
+static inline
+void lib_ring_buffer_reserve_push_reader(struct lib_ring_buffer *buf,
+					 struct channel *chan,
+					 unsigned long offset)
+{
+	unsigned long consumed_old, consumed_new;
+
+	do {
+		consumed_old = atomic_long_read(&buf->consumed);
+		/*
+		 * If buffer is in overwrite mode, push the reader consumed
+		 * count if the write position has reached it and we are not
+		 * at the first iteration (don't push the reader farther than
+		 * the writer). This operation can be done concurrently by many
+		 * writers in the same buffer, the writer being at the farthest
+		 * write position sub-buffer index in the buffer being the one
+		 * which will win this loop.
+		 */
+		if (unlikely(subbuf_trunc(offset, chan)
+			      - subbuf_trunc(consumed_old, chan)
+			     >= chan->backend.buf_size))
+			consumed_new = subbuf_align(consumed_old, chan);
+		else
+			return;
+	} while (unlikely(atomic_long_cmpxchg(&buf->consumed, consumed_old,
+					      consumed_new) != consumed_old));
+}
+
+static inline
+void lib_ring_buffer_vmcore_check_deliver(const struct lib_ring_buffer_config *config,
+					  struct lib_ring_buffer *buf,
+				          unsigned long commit_count,
+				          unsigned long idx)
+{
+	if (config->oops == RING_BUFFER_OOPS_CONSISTENCY)
+		v_set(config, &buf->commit_hot[idx].seq, commit_count);
+}
+
+static inline
+int lib_ring_buffer_poll_deliver(const struct lib_ring_buffer_config *config,
+				 struct lib_ring_buffer *buf,
+			         struct channel *chan)
+{
+	unsigned long consumed_old, consumed_idx, commit_count, write_offset;
+
+	consumed_old = atomic_long_read(&buf->consumed);
+	consumed_idx = subbuf_index(consumed_old, chan);
+	commit_count = v_read(config, &buf->commit_cold[consumed_idx].cc_sb);
+	/*
+	 * No memory barrier here, since we are only interested
+	 * in a statistically correct polling result. The next poll will
+	 * get the data is we are racing. The mb() that ensures correct
+	 * memory order is in get_subbuf.
+	 */
+	write_offset = v_read(config, &buf->offset);
+
+	/*
+	 * Check that the subbuffer we are trying to consume has been
+	 * already fully committed.
+	 */
+
+	if (((commit_count - chan->backend.subbuf_size)
+	     & chan->commit_count_mask)
+	    - (buf_trunc(consumed_old, chan)
+	       >> chan->backend.num_subbuf_order)
+	    != 0)
+		return 0;
+
+	/*
+	 * Check that we are not about to read the same subbuffer in
+	 * which the writer head is.
+	 */
+	if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_old, chan)
+	    == 0)
+		return 0;
+
+	return 1;
+
+}
+
+static inline
+int lib_ring_buffer_pending_data(const struct lib_ring_buffer_config *config,
+				 struct lib_ring_buffer *buf,
+				 struct channel *chan)
+{
+	return !!subbuf_offset(v_read(config, &buf->offset), chan);
+}
+
+static inline
+unsigned long lib_ring_buffer_get_data_size(const struct lib_ring_buffer_config *config,
+					    struct lib_ring_buffer *buf,
+					    unsigned long idx)
+{
+	return subbuffer_get_data_size(config, &buf->backend, idx);
+}
+
+/*
+ * Check if all space reservation in a buffer have been committed. This helps
+ * knowing if an execution context is nested (for per-cpu buffers only).
+ * This is a very specific ftrace use-case, so we keep this as "internal" API.
+ */
+static inline
+int lib_ring_buffer_reserve_committed(const struct lib_ring_buffer_config *config,
+				      struct lib_ring_buffer *buf,
+				      struct channel *chan)
+{
+	unsigned long offset, idx, commit_count;
+
+	CHAN_WARN_ON(chan, config->alloc != RING_BUFFER_ALLOC_PER_CPU);
+	CHAN_WARN_ON(chan, config->sync != RING_BUFFER_SYNC_PER_CPU);
+
+	/*
+	 * Read offset and commit count in a loop so they are both read
+	 * atomically wrt interrupts. By deal with interrupt concurrency by
+	 * restarting both reads if the offset has been pushed. Note that given
+	 * we only have to deal with interrupt concurrency here, an interrupt
+	 * modifying the commit count will also modify "offset", so it is safe
+	 * to only check for offset modifications.
+	 */
+	do {
+		offset = v_read(config, &buf->offset);
+		idx = subbuf_index(offset, chan);
+		commit_count = v_read(config, &buf->commit_hot[idx].cc);
+	} while (offset != v_read(config, &buf->offset));
+
+	return ((buf_trunc(offset, chan) >> chan->backend.num_subbuf_order)
+		     - (commit_count & chan->commit_count_mask) == 0);
+}
+
+static inline
+void lib_ring_buffer_check_deliver(const struct lib_ring_buffer_config *config,
+				   struct lib_ring_buffer *buf,
+			           struct channel *chan,
+			           unsigned long offset,
+				   unsigned long commit_count,
+			           unsigned long idx)
+{
+	unsigned long old_commit_count = commit_count
+					 - chan->backend.subbuf_size;
+	u64 tsc;
+
+	/* Check if all commits have been done */
+	if (unlikely((buf_trunc(offset, chan) >> chan->backend.num_subbuf_order)
+		     - (old_commit_count & chan->commit_count_mask) == 0)) {
+		/*
+		 * If we succeeded at updating cc_sb below, we are the subbuffer
+		 * writer delivering the subbuffer. Deals with concurrent
+		 * updates of the "cc" value without adding a add_return atomic
+		 * operation to the fast path.
+		 *
+		 * We are doing the delivery in two steps:
+		 * - First, we cmpxchg() cc_sb to the new value
+		 *   old_commit_count + 1. This ensures that we are the only
+		 *   subbuffer user successfully filling the subbuffer, but we
+		 *   do _not_ set the cc_sb value to "commit_count" yet.
+		 *   Therefore, other writers that would wrap around the ring
+		 *   buffer and try to start writing to our subbuffer would
+		 *   have to drop records, because it would appear as
+		 *   non-filled.
+		 *   We therefore have exclusive access to the subbuffer control
+		 *   structures.  This mutual exclusion with other writers is
+		 *   crucially important to perform record overruns count in
+		 *   flight recorder mode locklessly.
+		 * - When we are ready to release the subbuffer (either for
+		 *   reading or for overrun by other writers), we simply set the
+		 *   cc_sb value to "commit_count" and perform delivery.
+		 *
+		 * The subbuffer size is least 2 bytes (minimum size: 1 page).
+		 * This guarantees that old_commit_count + 1 != commit_count.
+		 */
+		if (likely(v_cmpxchg(config, &buf->commit_cold[idx].cc_sb,
+					 old_commit_count, old_commit_count + 1)
+			   == old_commit_count)) {
+			/*
+			 * Start of exclusive subbuffer access. We are
+			 * guaranteed to be the last writer in this subbuffer
+			 * and any other writer trying to access this subbuffer
+			 * in this state is required to drop records.
+			 */
+			tsc = config->cb.ring_buffer_clock_read(chan);
+			v_add(config,
+			      subbuffer_get_records_count(config,
+							  &buf->backend, idx),
+			      &buf->records_count);
+			v_add(config,
+			      subbuffer_count_records_overrun(config,
+							      &buf->backend,
+							      idx),
+			      &buf->records_overrun);
+			config->cb.buffer_end(buf, tsc, idx,
+					      lib_ring_buffer_get_data_size(config,
+									buf,
+									idx));
+
+			/*
+			 * Set noref flag and offset for this subbuffer id.
+			 * Contains a memory barrier that ensures counter stores
+			 * are ordered before set noref and offset.
+			 */
+			lib_ring_buffer_set_noref_offset(config, &buf->backend, idx,
+							 buf_trunc_val(offset, chan));
+
+			/*
+			 * Order set_noref and record counter updates before the
+			 * end of subbuffer exclusive access. Orders with
+			 * respect to writers coming into the subbuffer after
+			 * wrap around, and also order wrt concurrent readers.
+			 */
+			smp_mb();
+			/* End of exclusive subbuffer access */
+			v_set(config, &buf->commit_cold[idx].cc_sb,
+			      commit_count);
+			lib_ring_buffer_vmcore_check_deliver(config, buf,
+							 commit_count, idx);
+
+			/*
+			 * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
+			 */
+			if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
+			    && atomic_long_read(&buf->active_readers)
+			    && lib_ring_buffer_poll_deliver(config, buf, chan)) {
+				wake_up_interruptible(&buf->read_wait);
+				wake_up_interruptible(&chan->read_wait);
+			}
+
+		}
+	}
+}
+
+/*
+ * lib_ring_buffer_write_commit_counter
+ *
+ * For flight recording. must be called after commit.
+ * This function increments the subbuffer's commit_seq counter each time the
+ * commit count reaches back the reserve offset (modulo subbuffer size). It is
+ * useful for crash dump.
+ */
+static inline
+void lib_ring_buffer_write_commit_counter(const struct lib_ring_buffer_config *config,
+					  struct lib_ring_buffer *buf,
+				          struct channel *chan,
+				          unsigned long idx,
+				          unsigned long buf_offset,
+				          unsigned long commit_count,
+				          size_t slot_size)
+{
+	unsigned long offset, commit_seq_old;
+
+	if (config->oops != RING_BUFFER_OOPS_CONSISTENCY)
+		return;
+
+	offset = buf_offset + slot_size;
+
+	/*
+	 * subbuf_offset includes commit_count_mask. We can simply
+	 * compare the offsets within the subbuffer without caring about
+	 * buffer full/empty mismatch because offset is never zero here
+	 * (subbuffer header and record headers have non-zero length).
+	 */
+	if (unlikely(subbuf_offset(offset - commit_count, chan)))
+		return;
+
+	commit_seq_old = v_read(config, &buf->commit_hot[idx].seq);
+	while ((long) (commit_seq_old - commit_count) < 0)
+		commit_seq_old = v_cmpxchg(config, &buf->commit_hot[idx].seq,
+					   commit_seq_old, commit_count);
+}
+
+extern int lib_ring_buffer_create(struct lib_ring_buffer *buf,
+				  struct channel_backend *chanb, int cpu);
+extern void lib_ring_buffer_free(struct lib_ring_buffer *buf);
+
+/* Keep track of trap nesting inside ring buffer code */
+DECLARE_PER_CPU(unsigned int, lib_ring_buffer_nesting);
+
+#endif /* _LINUX_RING_BUFFER_FRONTEND_INTERNAL_H */

drivers/staging/lttng/lib/ringbuffer/frontend_types.h

+#ifndef _LINUX_RING_BUFFER_FRONTEND_TYPES_H
+#define _LINUX_RING_BUFFER_FRONTEND_TYPES_H
+
+/*
+ * linux/ringbuffer/frontend_types.h
+ *
+ * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring Buffer Library Synchronization Header (types).
+ *
+ * Author:
+ *	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * See ring_buffer_frontend.c for more information on wait-free algorithms.
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include <linux/kref.h>
+#include "../../wrapper/ringbuffer/config.h"
+#include "../../wrapper/ringbuffer/backend_types.h"
+#include "../../wrapper/spinlock.h"
+#include "../../lib/prio_heap/lttng_prio_heap.h"	/* For per-CPU read-side iterator */
+
+/*
+ * A switch is done during tracing or as a final flush after tracing (so it
+ * won't write in the new sub-buffer).
+ */
+enum switch_mode { SWITCH_ACTIVE, SWITCH_FLUSH };
+
+/* channel-level read-side iterator */
+struct channel_iter {
+	/* Prio heap of buffers. Lowest timestamps at the top. */
+	struct lttng_ptr_heap heap;	/* Heap of struct lib_ring_buffer ptrs */
+	struct list_head empty_head;	/* Empty buffers linked-list head */
+	int read_open;			/* Opened for reading ? */
+	u64 last_qs;			/* Last quiescent state timestamp */
+	u64 last_timestamp;		/* Last timestamp (for WARN_ON) */
+	int last_cpu;			/* Last timestamp cpu */
+	/*
+	 * read() file operation state.
+	 */
+	unsigned long len_left;
+};
+
+/* channel: collection of per-cpu ring buffers. */
+struct channel {
+	atomic_t record_disabled;
+	unsigned long commit_count_mask;	/*
+						 * Commit count mask, removing
+						 * the MSBs corresponding to
+						 * bits used to represent the
+						 * subbuffer index.
+						 */
+
+	struct channel_backend backend;		/* Associated backend */
+
+	unsigned long switch_timer_interval;	/* Buffer flush (jiffies) */
+	unsigned long read_timer_interval;	/* Reader wakeup (jiffies) */
+	struct notifier_block cpu_hp_notifier;	/* CPU hotplug notifier */
+	struct notifier_block tick_nohz_notifier; /* CPU nohz notifier */
+	struct notifier_block hp_iter_notifier;	/* hotplug iterator notifier */
+	int cpu_hp_enable:1;			/* Enable CPU hotplug notif. */
+	int hp_iter_enable:1;			/* Enable hp iter notif. */
+	wait_queue_head_t read_wait;		/* reader wait queue */
+	wait_queue_head_t hp_wait;		/* CPU hotplug wait queue */
+	int finalized;				/* Has channel been finalized */
+	struct channel_iter iter;		/* Channel read-side iterator */
+	struct kref ref;			/* Reference count */
+};
+
+/* Per-subbuffer commit counters used on the hot path */
+struct commit_counters_hot {
+	union v_atomic cc;		/* Commit counter */
+	union v_atomic seq;		/* Consecutive commits */
+};
+
+/* Per-subbuffer commit counters used only on cold paths */
+struct commit_counters_cold {
+	union v_atomic cc_sb;		/* Incremented _once_ at sb switch */
+};
+
+/* Per-buffer read iterator */
+struct lib_ring_buffer_iter {
+	u64 timestamp;			/* Current record timestamp */
+	size_t header_len;		/* Current record header length */
+	size_t payload_len;		/* Current record payload length */
+
+	struct list_head empty_node;	/* Linked list of empty buffers */
+	unsigned long consumed, read_offset, data_size;
+	enum {
+		ITER_GET_SUBBUF = 0,
+		ITER_TEST_RECORD,
+		ITER_NEXT_RECORD,
+		ITER_PUT_SUBBUF,
+	} state;
+	int allocated:1;
+	int read_open:1;		/* Opened for reading ? */
+};
+
+/* ring buffer state */
+struct lib_ring_buffer {
+	/* First 32 bytes cache-hot cacheline */
+	union v_atomic offset;		/* Current offset in the buffer */
+	struct commit_counters_hot *commit_hot;
+					/* Commit count per sub-buffer */
+	atomic_long_t consumed;		/*
+					 * Current offset in the buffer
+					 * standard atomic access (shared)
+					 */
+	atomic_t record_disabled;
+	/* End of first 32 bytes cacheline */
+	union v_atomic last_tsc;	/*
+					 * Last timestamp written in the buffer.
+					 */
+
+	struct lib_ring_buffer_backend backend;	/* Associated backend */
+
+	struct commit_counters_cold *commit_cold;
+					/* Commit count per sub-buffer */
+	atomic_long_t active_readers;	/*
+					 * Active readers count
+					 * standard atomic access (shared)
+					 */
+					/* Dropped records */
+	union v_atomic records_lost_full;	/* Buffer full */
+	union v_atomic records_lost_wrap;	/* Nested wrap-around */
+	union v_atomic records_lost_big;	/* Events too big */
+	union v_atomic records_count;	/* Number of records written */
+	union v_atomic records_overrun;	/* Number of overwritten records */
+	wait_queue_head_t read_wait;	/* reader buffer-level wait queue */
+	wait_queue_head_t write_wait;	/* writer buffer-level wait queue (for metadata only) */
+	int finalized;			/* buffer has been finalized */
+	struct timer_list switch_timer;	/* timer for periodical switch */
+	struct timer_list read_timer;	/* timer for read poll */
+	raw_spinlock_t raw_tick_nohz_spinlock;	/* nohz entry lock/trylock */
+	struct lib_ring_buffer_iter iter;	/* read-side iterator */
+	unsigned long get_subbuf_consumed;	/* Read-side consumed */
+	unsigned long prod_snapshot;	/* Producer count snapshot */
+	unsigned long cons_snapshot;	/* Consumer count snapshot */
+	int get_subbuf:1;		/* Sub-buffer being held by reader */
+	int switch_timer_enabled:1;	/* Protected by ring_buffer_nohz_lock */
+	int read_timer_enabled:1;	/* Protected by ring_buffer_nohz_lock */
+};
+
+static inline
+void *channel_get_private(struct channel *chan)
+{
+	return chan->backend.priv;
+}
+
+/*
+ * Issue warnings and disable channels upon internal error.
+ * Can receive struct lib_ring_buffer or struct lib_ring_buffer_backend
+ * parameters.
+ */
+#define CHAN_WARN_ON(c, cond)						\
+	({								\
+		struct channel *__chan;					\
+		int _____ret = unlikely(cond);				\
+		if (_____ret) {						\
+			if (__same_type(*(c), struct channel_backend))	\
+				__chan = container_of((void *) (c),	\
+							struct channel, \
+							backend);	\
+			else if (__same_type(*(c), struct channel))	\
+				__chan = (void *) (c);			\
+			else						\
+				BUG_ON(1);				\
+			atomic_inc(&__chan->record_disabled);		\
+			WARN_ON(1);					\
+		}							\
+		_____ret;						\
+	})
+
+#endif /* _LINUX_RING_BUFFER_FRONTEND_TYPES_H */

drivers/staging/lttng/lib/ringbuffer/iterator.h

+#ifndef _LINUX_RING_BUFFER_ITERATOR_H
+#define _LINUX_RING_BUFFER_ITERATOR_H
+
+/*
+ * linux/ringbuffer/iterator.h
+ *
+ * (C) Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Ring buffer and channel iterators.
+ *
+ * Author:
+ *	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#include "../../wrapper/ringbuffer/backend.h"
+#include "../../wrapper/ringbuffer/frontend.h"
+
+/*
+ * lib_ring_buffer_get_next_record advances the buffer read position to the next
+ * record. It returns either the size of the next record, -EAGAIN if there is
+ * currently no data available, or -ENODATA if no data is available and buffer
+ * is finalized.
+ */
+extern ssize_t lib_ring_buffer_get_next_record(struct channel *chan,
+					       struct lib_ring_buffer *buf);
+
+/*
+ * channel_get_next_record advances the buffer read position to the next record.
+ * It returns either the size of the next record, -EAGAIN if there is currently
+ * no data available, or -ENODATA if no data is available and buffer is
+ * finalized.
+ * Returns the current buffer in ret_buf.
+ */
+extern ssize_t channel_get_next_record(struct channel *chan,
+				       struct lib_ring_buffer **ret_buf);
+
+/**
+ * read_current_record - copy the buffer current record into dest.
+ * @buf: ring buffer
+ * @dest: destination where the record should be copied
+ *
+ * dest should be large enough to contain the record. Returns the number of
+ * bytes copied.
+ */
+static inline size_t read_current_record(struct lib_ring_buffer *buf, void *dest)
+{
+	return lib_ring_buffer_read(&buf->backend, buf->iter.read_offset,
+				    dest, buf->iter.payload_len);
+}
+
+extern int lib_ring_buffer_iterator_open(struct lib_ring_buffer *buf);
+extern void lib_ring_buffer_iterator_release(struct lib_ring_buffer *buf);
+extern int channel_iterator_open(struct channel *chan);
+extern void channel_iterator_release(struct channel *chan);
+
+extern const struct file_operations channel_payload_file_operations;
+extern const struct file_operations lib_ring_buffer_payload_file_operations;
+
+/*
+ * Used internally.
+ */
+int channel_iterator_init(struct channel *chan);
+void channel_iterator_unregister_notifiers(struct channel *chan);
+void channel_iterator_free(struct channel *chan);
+void channel_iterator_reset(struct channel *chan);
+void lib_ring_buffer_iterator_reset(struct lib_ring_buffer *buf);
+
+#endif /* _LINUX_RING_BUFFER_ITERATOR_H */

drivers/staging/lttng/lib/ringbuffer/nohz.h

+#ifndef _LINUX_RING_BUFFER_NOHZ_H
+#define _LINUX_RING_BUFFER_NOHZ_H
+
+/*
+ * ringbuffer/nohz.h
+ *
+ * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * Dual LGPL v2.1/GPL v2 license.
+ */
+
+#ifdef CONFIG_LIB_RING_BUFFER
+void lib_ring_buffer_tick_nohz_flush(void);
+void lib_ring_buffer_tick_nohz_stop(void);
+void lib_ring_buffer_tick_nohz_restart(void);
+#else
+static inline void lib_ring_buffer_tick_nohz_flush(void)
+{
+}
+
+static inline void lib_ring_buffer_tick_nohz_stop(void)
+{
+}
+
+static inline void lib_ring_buffer_tick_nohz_restart(void)
+{
+}
+#endif
+
+#endif /* _LINUX_RING_BUFFER_NOHZ_H */