inode_mark.c

/*
 *  Copyright (C) 2008 Red Hat, Inc., Eric Paris <eparis@redhat.com>
 *
 *  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, 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; see the file COPYING.  If not, write to
 *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 */

/*
 * fsnotify inode mark locking/lifetime/and refcnting
 *
 * REFCNT:
 * The mark->refcnt tells how many "things" in the kernel currently are
 * referencing this object.  The object typically will live inside the kernel
 * with a refcnt of 2, one for each list it is on (i_list, g_list).  Any task
 * which can find this object holding the appropriete locks, can take a reference
 * and the object itself is guarenteed to survive until the reference is dropped.
 *
 * LOCKING:
 * There are 3 spinlocks involved with fsnotify inode marks and they MUST
 * be taken in order as follows:
 *
 * entry->lock
 * group->mark_lock
 * inode->i_lock
 *
 * entry->lock protects 2 things, entry->group and entry->inode.  You must hold
 * that lock to dereference either of these things (they could be NULL even with
 * the lock)
 *
 * group->mark_lock protects the mark_entries list anchored inside a given group
 * and each entry is hooked via the g_list.  It also sorta protects the
 * free_g_list, which when used is anchored by a private list on the stack of the
 * task which held the group->mark_lock.
 *
 * inode->i_lock protects the i_fsnotify_mark_entries list anchored inside a
 * given inode and each entry is hooked via the i_list. (and sorta the
 * free_i_list)
 *
 *
 * LIFETIME:
 * Inode marks survive between when they are added to an inode and when their
 * refcnt==0.
 *
 * The inode mark can be cleared for a number of different reasons including:
 * - The inode is unlinked for the last time.  (fsnotify_inode_remove)
 * - The inode is being evicted from cache. (fsnotify_inode_delete)
 * - The fs the inode is on is unmounted.  (fsnotify_inode_delete/fsnotify_unmount_inodes)
 * - Something explicitly requests that it be removed.  (fsnotify_destroy_mark_by_entry)
 * - The fsnotify_group associated with the mark is going away and all such marks
 *   need to be cleaned up. (fsnotify_clear_marks_by_group)
 *
 * Worst case we are given an inode and need to clean up all the marks on that
 * inode.  We take i_lock and walk the i_fsnotify_mark_entries safely.  For each
 * mark on the list we take a reference (so the mark can't disappear under us).
 * We remove that mark form the inode's list of marks and we add this mark to a
 * private list anchored on the stack using i_free_list;  At this point we no
 * longer fear anything finding the mark using the inode's list of marks.
 *
 * We can safely and locklessly run the private list on the stack of everything
 * we just unattached from the original inode.  For each mark on the private list
 * we grab the mark-> and can thus dereference mark->group and mark->inode.  If
 * we see the group and inode are not NULL we take those locks.  Now holding all
 * 3 locks we can completely remove the mark from other tasks finding it in the
 * future.  Remember, 10 things might already be referencing this mark, but they
 * better be holding a ref.  We drop our reference we took before we unhooked it
 * from the inode.  When the ref hits 0 we can free the mark.
 *
 * Very similarly for freeing by group, except we use free_g_list.
 *
 * This has the very interesting property of being able to run concurrently with
 * any (or all) other directions.
 */

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/spinlock.h>

#include <asm/atomic.h>

#include <linux/fsnotify_backend.h>
#include "fsnotify.h"

void fsnotify_get_mark(struct fsnotify_mark_entry *entry)
{
	atomic_inc(&entry->refcnt);
}

void fsnotify_put_mark(struct fsnotify_mark_entry *entry)
{
	if (atomic_dec_and_test(&entry->refcnt))
		entry->free_mark(entry);
}

/*
 * Recalculate the mask of events relevant to a given inode locked.
 */
static void fsnotify_recalc_inode_mask_locked(struct inode *inode)
{
	struct fsnotify_mark_entry *entry;
	struct hlist_node *pos;
	__u32 new_mask = 0;

	assert_spin_locked(&inode->i_lock);

	hlist_for_each_entry(entry, pos, &inode->i_fsnotify_mark_entries, i_list)
		new_mask |= entry->mask;
	inode->i_fsnotify_mask = new_mask;
}

/*
 * Recalculate the inode->i_fsnotify_mask, or the mask of all FS_* event types
 * any notifier is interested in hearing for this inode.
 */
void fsnotify_recalc_inode_mask(struct inode *inode)
{
	spin_lock(&inode->i_lock);
	fsnotify_recalc_inode_mask_locked(inode);
	spin_unlock(&inode->i_lock);
}

/*
 * Any time a mark is getting freed we end up here.
 * The caller had better be holding a reference to this mark so we don't actually
 * do the final put under the entry->lock
 */
void fsnotify_destroy_mark_by_entry(struct fsnotify_mark_entry *entry)
{
	struct fsnotify_group *group;
	struct inode *inode;

	spin_lock(&entry->lock);

	group = entry->group;
	inode = entry->inode;

	BUG_ON(group && !inode);
	BUG_ON(!group && inode);

	/* if !group something else already marked this to die */
	if (!group) {
		spin_unlock(&entry->lock);
		return;
	}

	/* 1 from caller and 1 for being on i_list/g_list */
	BUG_ON(atomic_read(&entry->refcnt) < 2);

	spin_lock(&group->mark_lock);
	spin_lock(&inode->i_lock);

	hlist_del_init(&entry->i_list);
	entry->inode = NULL;

	list_del_init(&entry->g_list);
	entry->group = NULL;

	fsnotify_put_mark(entry); /* for i_list and g_list */

	/*
	 * this mark is now off the inode->i_fsnotify_mark_entries list and we
	 * hold the inode->i_lock, so this is the perfect time to update the
	 * inode->i_fsnotify_mask
	 */
	fsnotify_recalc_inode_mask_locked(inode);

	spin_unlock(&inode->i_lock);
	spin_unlock(&group->mark_lock);
	spin_unlock(&entry->lock);

	/*
	 * Some groups like to know that marks are being freed.  This is a
	 * callback to the group function to let it know that this entry
	 * is being freed.
	 */
	group->ops->freeing_mark(entry, group);

	/*
	 * it's possible that this group tried to destroy itself, but this
	 * this mark was simultaneously being freed by inode.  If that's the
	 * case, we finish freeing the group here.
	 */
	if (unlikely(atomic_dec_and_test(&group->num_marks)))
		fsnotify_final_destroy_group(group);
}

/*
 * Given a group, destroy all of the marks associated with that group.
 */
void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
{
	struct fsnotify_mark_entry *lentry, *entry;
	LIST_HEAD(free_list);

	spin_lock(&group->mark_lock);
	list_for_each_entry_safe(entry, lentry, &group->mark_entries, g_list) {
		list_add(&entry->free_g_list, &free_list);
		list_del_init(&entry->g_list);
		fsnotify_get_mark(entry);
	}
	spin_unlock(&group->mark_lock);

	list_for_each_entry_safe(entry, lentry, &free_list, free_g_list) {
		fsnotify_destroy_mark_by_entry(entry);
		fsnotify_put_mark(entry);
	}
}

/*
 * Given an inode, destroy all of the marks associated with that inode.
 */
void fsnotify_clear_marks_by_inode(struct inode *inode)
{
	struct fsnotify_mark_entry *entry, *lentry;
	struct hlist_node *pos, *n;
	LIST_HEAD(free_list);

	spin_lock(&inode->i_lock);
	hlist_for_each_entry_safe(entry, pos, n, &inode->i_fsnotify_mark_entries, i_list) {
		list_add(&entry->free_i_list, &free_list);
		hlist_del_init(&entry->i_list);
		fsnotify_get_mark(entry);
	}
	spin_unlock(&inode->i_lock);

	list_for_each_entry_safe(entry, lentry, &free_list, free_i_list) {
		fsnotify_destroy_mark_by_entry(entry);
		fsnotify_put_mark(entry);
	}
}

/*
 * given a group and inode, find the mark associated with that combination.
 * if found take a reference to that mark and return it, else return NULL
 */
struct fsnotify_mark_entry *fsnotify_find_mark_entry(struct fsnotify_group *group,
						     struct inode *inode)
{
	struct fsnotify_mark_entry *entry;
	struct hlist_node *pos;

	assert_spin_locked(&inode->i_lock);

	hlist_for_each_entry(entry, pos, &inode->i_fsnotify_mark_entries, i_list) {
		if (entry->group == group) {
			fsnotify_get_mark(entry);
			return entry;
		}
	}
	return NULL;
}

/*
 * Nothing fancy, just initialize lists and locks and counters.
 */
void fsnotify_init_mark(struct fsnotify_mark_entry *entry,
			void (*free_mark)(struct fsnotify_mark_entry *entry))

{
	spin_lock_init(&entry->lock);
	atomic_set(&entry->refcnt, 1);
	INIT_HLIST_NODE(&entry->i_list);
	entry->group = NULL;
	entry->mask = 0;
	entry->inode = NULL;
	entry->free_mark = free_mark;
}

/*
 * Attach an initialized mark entry to a given group and inode.
 * These marks may be used for the fsnotify backend to determine which
 * event types should be delivered to which group and for which inodes.
 */
int fsnotify_add_mark(struct fsnotify_mark_entry *entry,
		      struct fsnotify_group *group, struct inode *inode)
{
	struct fsnotify_mark_entry *lentry;
	int ret = 0;

	/*
	 * LOCKING ORDER!!!!
	 * entry->lock
	 * group->mark_lock
	 * inode->i_lock
	 */
	spin_lock(&entry->lock);
	spin_lock(&group->mark_lock);
	spin_lock(&inode->i_lock);

	entry->group = group;
	entry->inode = inode;

	lentry = fsnotify_find_mark_entry(group, inode);
	if (!lentry) {
		hlist_add_head(&entry->i_list, &inode->i_fsnotify_mark_entries);
		list_add(&entry->g_list, &group->mark_entries);

		fsnotify_get_mark(entry); /* for i_list and g_list */

		atomic_inc(&group->num_marks);

		fsnotify_recalc_inode_mask_locked(inode);
	}

	spin_unlock(&inode->i_lock);
	spin_unlock(&group->mark_lock);
	spin_unlock(&entry->lock);

	if (lentry) {
		ret = -EEXIST;
		fsnotify_put_mark(lentry);
	}

	return ret;
}