printk_safe.c

/*
 * printk_safe.c - Safe printk for printk-deadlock-prone contexts
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/debug_locks.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
#include <linux/irq_work.h>
#include <linux/printk.h>

#include "internal.h"

/*
 * printk() could not take logbuf_lock in NMI context. Instead,
 * it uses an alternative implementation that temporary stores
 * the strings into a per-CPU buffer. The content of the buffer
 * is later flushed into the main ring buffer via IRQ work.
 *
 * The alternative implementation is chosen transparently
 * by examinig current printk() context mask stored in @printk_context
 * per-CPU variable.
 *
 * The implementation allows to flush the strings also from another CPU.
 * There are situations when we want to make sure that all buffers
 * were handled or when IRQs are blocked.
 */
static int printk_safe_irq_ready;
atomic_t nmi_message_lost;

#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
			 sizeof(atomic_t) - sizeof(struct irq_work))

struct printk_safe_seq_buf {
	atomic_t		len;	/* length of written data */
	struct irq_work		work;	/* IRQ work that flushes the buffer */
	unsigned char		buffer[SAFE_LOG_BUF_LEN];
};

static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
static DEFINE_PER_CPU(int, printk_context);

#ifdef CONFIG_PRINTK_NMI
static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
#endif

/*
 * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
 * have dedicated buffers, because otherwise printk-safe preempted by
 * NMI-printk would have overwritten the NMI messages.
 *
 * The messages are fushed from irq work (or from panic()), possibly,
 * from other CPU, concurrently with printk_safe_log_store(). Should this
 * happen, printk_safe_log_store() will notice the buffer->len mismatch
 * and repeat the write.
 */
static int printk_safe_log_store(struct printk_safe_seq_buf *s,
				 const char *fmt, va_list args)
{
	int add;
	size_t len;

again:
	len = atomic_read(&s->len);

	/* The trailing '\0' is not counted into len. */
	if (len >= sizeof(s->buffer) - 1) {
		atomic_inc(&nmi_message_lost);
		return 0;
	}

	/*
	 * Make sure that all old data have been read before the buffer
	 * was reset. This is not needed when we just append data.
	 */
	if (!len)
		smp_rmb();

	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);

	/*
	 * Do it once again if the buffer has been flushed in the meantime.
	 * Note that atomic_cmpxchg() is an implicit memory barrier that
	 * makes sure that the data were written before updating s->len.
	 */
	if (atomic_cmpxchg(&s->len, len, len + add) != len)
		goto again;

	/* Get flushed in a more safe context. */
	if (add && printk_safe_irq_ready) {
		/* Make sure that IRQ work is really initialized. */
		smp_rmb();
		irq_work_queue(&s->work);
	}

	return add;
}

static void printk_safe_flush_line(const char *text, int len)
{
	/*
	 * The buffers are flushed in NMI only on panic.  The messages must
	 * go only into the ring buffer at this stage.  Consoles will get
	 * explicitly called later when a crashdump is not generated.
	 */
	if (in_nmi())
		printk_deferred("%.*s", len, text);
	else
		printk("%.*s", len, text);
}

/* printk part of the temporary buffer line by line */
static int printk_safe_flush_buffer(const char *start, size_t len)
{
	const char *c, *end;
	bool header;

	c = start;
	end = start + len;
	header = true;

	/* Print line by line. */
	while (c < end) {
		if (*c == '\n') {
			printk_safe_flush_line(start, c - start + 1);
			start = ++c;
			header = true;
			continue;
		}

		/* Handle continuous lines or missing new line. */
		if ((c + 1 < end) && printk_get_level(c)) {
			if (header) {
				c = printk_skip_level(c);
				continue;
			}

			printk_safe_flush_line(start, c - start);
			start = c++;
			header = true;
			continue;
		}

		header = false;
		c++;
	}

	/* Check if there was a partial line. Ignore pure header. */
	if (start < end && !header) {
		static const char newline[] = KERN_CONT "\n";

		printk_safe_flush_line(start, end - start);
		printk_safe_flush_line(newline, strlen(newline));
	}

	return len;
}

/*
 * Flush data from the associated per-CPU buffer. The function
 * can be called either via IRQ work or independently.
 */
static void __printk_safe_flush(struct irq_work *work)
{
	static raw_spinlock_t read_lock =
		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
	struct printk_safe_seq_buf *s =
		container_of(work, struct printk_safe_seq_buf, work);
	unsigned long flags;
	size_t len;
	int i;

	/*
	 * The lock has two functions. First, one reader has to flush all
	 * available message to make the lockless synchronization with
	 * writers easier. Second, we do not want to mix messages from
	 * different CPUs. This is especially important when printing
	 * a backtrace.
	 */
	raw_spin_lock_irqsave(&read_lock, flags);

	i = 0;
more:
	len = atomic_read(&s->len);

	/*
	 * This is just a paranoid check that nobody has manipulated
	 * the buffer an unexpected way. If we printed something then
	 * @len must only increase. Also it should never overflow the
	 * buffer size.
	 */
	if ((i && i >= len) || len > sizeof(s->buffer)) {
		const char *msg = "printk_safe_flush: internal error\n";

		printk_safe_flush_line(msg, strlen(msg));
		len = 0;
	}

	if (!len)
		goto out; /* Someone else has already flushed the buffer. */

	/* Make sure that data has been written up to the @len */
	smp_rmb();
	i += printk_safe_flush_buffer(s->buffer + i, len - i);

	/*
	 * Check that nothing has got added in the meantime and truncate
	 * the buffer. Note that atomic_cmpxchg() is an implicit memory
	 * barrier that makes sure that the data were copied before
	 * updating s->len.
	 */
	if (atomic_cmpxchg(&s->len, len, 0) != len)
		goto more;

out:
	raw_spin_unlock_irqrestore(&read_lock, flags);
}

/**
 * printk_safe_flush - flush all per-cpu nmi buffers.
 *
 * The buffers are flushed automatically via IRQ work. This function
 * is useful only when someone wants to be sure that all buffers have
 * been flushed at some point.
 */
void printk_safe_flush(void)
{
	int cpu;

	for_each_possible_cpu(cpu) {
#ifdef CONFIG_PRINTK_NMI
		__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
#endif
		__printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
	}
}

/**
 * printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
 *	goes down.
 *
 * Similar to printk_safe_flush() but it can be called even in NMI context when
 * the system goes down. It does the best effort to get NMI messages into
 * the main ring buffer.
 *
 * Note that it could try harder when there is only one CPU online.
 */
void printk_safe_flush_on_panic(void)
{
	/*
	 * Make sure that we could access the main ring buffer.
	 * Do not risk a double release when more CPUs are up.
	 */
	if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
		if (num_online_cpus() > 1)
			return;

		debug_locks_off();
		raw_spin_lock_init(&logbuf_lock);
	}

	printk_safe_flush();
}

#ifdef CONFIG_PRINTK_NMI
/*
 * Safe printk() for NMI context. It uses a per-CPU buffer to
 * store the message. NMIs are not nested, so there is always only
 * one writer running. But the buffer might get flushed from another
 * CPU, so we need to be careful.
 */
static int vprintk_nmi(const char *fmt, va_list args)
{
	struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);

	return printk_safe_log_store(s, fmt, args);
}

void printk_nmi_enter(void)
{
	this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
}

void printk_nmi_exit(void)
{
	this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
}

#else

static int vprintk_nmi(const char *fmt, va_list args)
{
	return 0;
}

#endif /* CONFIG_PRINTK_NMI */

/*
 * Lock-less printk(), to avoid deadlocks should the printk() recurse
 * into itself. It uses a per-CPU buffer to store the message, just like
 * NMI.
 */
static int vprintk_safe(const char *fmt, va_list args)
{
	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);

	return printk_safe_log_store(s, fmt, args);
}

/* Can be preempted by NMI. */
void __printk_safe_enter(void)
{
	this_cpu_inc(printk_context);
}

/* Can be preempted by NMI. */
void __printk_safe_exit(void)
{
	this_cpu_dec(printk_context);
}

__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
{
	if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
		return vprintk_nmi(fmt, args);

	if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
		return vprintk_safe(fmt, args);

	return vprintk_default(fmt, args);
}

void __init printk_safe_init(void)
{
	int cpu;

	for_each_possible_cpu(cpu) {
		struct printk_safe_seq_buf *s;

		s = &per_cpu(safe_print_seq, cpu);
		init_irq_work(&s->work, __printk_safe_flush);

#ifdef CONFIG_PRINTK_NMI
		s = &per_cpu(nmi_print_seq, cpu);
		init_irq_work(&s->work, __printk_safe_flush);
#endif
	}

	/* Make sure that IRQ works are initialized before enabling. */
	smp_wmb();
	printk_safe_irq_ready = 1;

	/* Flush pending messages that did not have scheduled IRQ works. */
	printk_safe_flush();
}