fs/notify/mark.c - kernel/msm-5.4 - Gitiles

 /*
  *  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:
  *
  * mark->lock
  * group->mark_lock
  * inode->i_lock
  *
  * mark->lock protects 2 things, mark->group and mark->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 marks_list anchored inside a given group
  * and each mark 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_marks list anchored inside a
  * given inode and each mark 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)
  * - 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_marks 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/kthread.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/srcu.h>
 #include <linux/writeback.h> /* for inode_lock */

 #include <asm/atomic.h>

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

 struct srcu_struct fsnotify_mark_srcu;
 static DEFINE_SPINLOCK(destroy_lock);
 static LIST_HEAD(destroy_list);
 static DECLARE_WAIT_QUEUE_HEAD(destroy_waitq);

 void fsnotify_get_mark(struct fsnotify_mark *mark)
 {
 	atomic_inc(&mark->refcnt);
 }

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

 /*
  * 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 mark->lock
  */
 void fsnotify_destroy_mark(struct fsnotify_mark *mark)
 {
 	struct fsnotify_group *group;
 	struct inode *inode = NULL;

 	spin_lock(&mark->lock);

 	group = mark->group;

 	/* something else already called this function on this mark */
 	if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
 		spin_unlock(&mark->lock);
 		return;
 	}

 	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;

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

 	spin_lock(&group->mark_lock);

 	if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
 		inode = mark->i.inode;
 		fsnotify_destroy_inode_mark(mark);
 	} else if (mark->flags & FSNOTIFY_MARK_FLAG_VFSMOUNT)
 		fsnotify_destroy_vfsmount_mark(mark);
 	else
 		BUG();

 	list_del_init(&mark->g_list);

 	spin_unlock(&group->mark_lock);
 	spin_unlock(&mark->lock);

 	spin_lock(&destroy_lock);
 	list_add(&mark->destroy_list, &destroy_list);
 	spin_unlock(&destroy_lock);
 	wake_up(&destroy_waitq);

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

 	/*
 	 * __fsnotify_update_child_dentry_flags(inode);
 	 *
 	 * I really want to call that, but we can't, we have no idea if the inode
 	 * still exists the second we drop the mark->lock.
 	 *
 	 * The next time an event arrive to this inode from one of it's children
 	 * __fsnotify_parent will see that the inode doesn't care about it's
 	 * children and will update all of these flags then.  So really this
 	 * is just a lazy update (and could be a perf win...)
 	 */

 	if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
 		iput(inode);

 	/*
 	 * 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);
 }

 void fsnotify_set_mark_mask_locked(struct fsnotify_mark *mark, __u32 mask)
 {
 	assert_spin_locked(&mark->lock);

 	mark->mask = mask;

 	if (mark->flags & FSNOTIFY_MARK_FLAG_INODE)
 		fsnotify_set_inode_mark_mask_locked(mark, mask);
 }

 void fsnotify_set_mark_ignored_mask_locked(struct fsnotify_mark *mark, __u32 mask)
 {
 	assert_spin_locked(&mark->lock);

 	mark->ignored_mask = mask;
 }

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

 	BUG_ON(inode && mnt);
 	BUG_ON(!inode && !mnt);

 	/*
 	 * if this group isn't being testing for inode type events we need
 	 * to start testing
 	 */
 	if (inode && unlikely(list_empty(&group->inode_group_list)))
 		fsnotify_add_inode_group(group);
 	else if (mnt && unlikely(list_empty(&group->vfsmount_group_list)))
 		fsnotify_add_vfsmount_group(group);

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

 	mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE;

 	mark->group = group;
 	list_add(&mark->g_list, &group->marks_list);
 	atomic_inc(&group->num_marks);
 	fsnotify_get_mark(mark); /* for i_list and g_list */

 	if (inode) {
 		ret = fsnotify_add_inode_mark(mark, group, inode, allow_dups);
 		if (ret)
 			goto err;
 	} else if (mnt) {
 		ret = fsnotify_add_vfsmount_mark(mark, group, mnt, allow_dups);
 		if (ret)
 			goto err;
 	} else {
 		BUG();
 	}

 	spin_unlock(&group->mark_lock);

 	/* this will pin the object if appropriate */
 	fsnotify_set_mark_mask_locked(mark, mark->mask);

 	spin_unlock(&mark->lock);

 	if (inode)
 		__fsnotify_update_child_dentry_flags(inode);

 	return ret;
 err:
 	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
 	list_del_init(&mark->g_list);
 	mark->group = NULL;
 	atomic_dec(&group->num_marks);

 	spin_unlock(&group->mark_lock);
 	spin_unlock(&mark->lock);

 	spin_lock(&destroy_lock);
 	list_add(&mark->destroy_list, &destroy_list);
 	spin_unlock(&destroy_lock);
 	wake_up(&destroy_waitq);

 	return ret;
 }

 /*
  * clear any marks in a group in which mark->flags & flags is true
  */
 void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
 					 unsigned int flags)
 {
 	struct fsnotify_mark *lmark, *mark;
 	LIST_HEAD(free_list);

 	spin_lock(&group->mark_lock);
 	list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
 		if (mark->flags & flags) {
 			list_add(&mark->free_g_list, &free_list);
 			list_del_init(&mark->g_list);
 			fsnotify_get_mark(mark);
 		}
 	}
 	spin_unlock(&group->mark_lock);

 	list_for_each_entry_safe(mark, lmark, &free_list, free_g_list) {
 		fsnotify_destroy_mark(mark);
 		fsnotify_put_mark(mark);
 	}
 }

 /*
  * Given a group, destroy all of the marks associated with that group.
  */
 void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
 {
 	fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1);
 }

 void fsnotify_duplicate_mark(struct fsnotify_mark *new, struct fsnotify_mark *old)
 {
 	assert_spin_locked(&old->lock);
 	new->i.inode = old->i.inode;
 	new->m.mnt = old->m.mnt;
 	new->group = old->group;
 	new->mask = old->mask;
 	new->free_mark = old->free_mark;
 }

 /*
  * Nothing fancy, just initialize lists and locks and counters.
  */
 void fsnotify_init_mark(struct fsnotify_mark *mark,
 			void (*free_mark)(struct fsnotify_mark *mark))
 {
 	memset(mark, 0, sizeof(*mark));
 	spin_lock_init(&mark->lock);
 	atomic_set(&mark->refcnt, 1);
 	mark->free_mark = free_mark;
 }

 static int fsnotify_mark_destroy(void *ignored)
 {
 	struct fsnotify_mark *mark, *next;
 	LIST_HEAD(private_destroy_list);

 	for (;;) {
 		spin_lock(&destroy_lock);
 		/* exchange the list head */
 		list_replace_init(&destroy_list, &private_destroy_list);
 		spin_unlock(&destroy_lock);

 		synchronize_srcu(&fsnotify_mark_srcu);

 		list_for_each_entry_safe(mark, next, &private_destroy_list, destroy_list) {
 			list_del_init(&mark->destroy_list);
 			fsnotify_put_mark(mark);
 		}

 		wait_event_interruptible(destroy_waitq, !list_empty(&destroy_list));
 	}

 	return 0;
 }

 static int __init fsnotify_mark_init(void)
 {
 	struct task_struct *thread;

 	thread = kthread_run(fsnotify_mark_destroy, NULL,
 			     "fsnotify_mark");
 	if (IS_ERR(thread))
 		panic("unable to start fsnotify mark destruction thread.");

 	return 0;
 }
 device_initcall(fsnotify_mark_init);
	/*
	* 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:
	*
	* mark->lock
	* group->mark_lock
	* inode->i_lock
	*
	* mark->lock protects 2 things, mark->group and mark->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 marks_list anchored inside a given group
	* and each mark 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_marks list anchored inside a
	* given inode and each mark 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)
	* - 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_marks 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/kthread.h>
	#include <linux/module.h>
	#include <linux/mutex.h>
	#include <linux/slab.h>
	#include <linux/spinlock.h>
	#include <linux/srcu.h>
	#include <linux/writeback.h> /* for inode_lock */

	#include <asm/atomic.h>

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

	struct srcu_struct fsnotify_mark_srcu;
	static DEFINE_SPINLOCK(destroy_lock);
	static LIST_HEAD(destroy_list);
	static DECLARE_WAIT_QUEUE_HEAD(destroy_waitq);

	void fsnotify_get_mark(struct fsnotify_mark *mark)
	{
	atomic_inc(&mark->refcnt);
	}

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

	/*
	* 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 mark->lock
	*/
	void fsnotify_destroy_mark(struct fsnotify_mark *mark)
	{
	struct fsnotify_group *group;
	struct inode *inode = NULL;

	spin_lock(&mark->lock);

	group = mark->group;

	/* something else already called this function on this mark */
	if (!(mark->flags & FSNOTIFY_MARK_FLAG_ALIVE)) {
	spin_unlock(&mark->lock);
	return;
	}

	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;

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

	spin_lock(&group->mark_lock);

	if (mark->flags & FSNOTIFY_MARK_FLAG_INODE) {
	inode = mark->i.inode;
	fsnotify_destroy_inode_mark(mark);
	} else if (mark->flags & FSNOTIFY_MARK_FLAG_VFSMOUNT)
	fsnotify_destroy_vfsmount_mark(mark);
	else
	BUG();

	list_del_init(&mark->g_list);

	spin_unlock(&group->mark_lock);
	spin_unlock(&mark->lock);

	spin_lock(&destroy_lock);
	list_add(&mark->destroy_list, &destroy_list);
	spin_unlock(&destroy_lock);
	wake_up(&destroy_waitq);

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

	/*
	* __fsnotify_update_child_dentry_flags(inode);
	*
	* I really want to call that, but we can't, we have no idea if the inode
	* still exists the second we drop the mark->lock.
	*
	* The next time an event arrive to this inode from one of it's children
	* __fsnotify_parent will see that the inode doesn't care about it's
	* children and will update all of these flags then. So really this
	* is just a lazy update (and could be a perf win...)
	*/

	if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
	iput(inode);

	/*
	* 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);
	}

	void fsnotify_set_mark_mask_locked(struct fsnotify_mark *mark, __u32 mask)
	{
	assert_spin_locked(&mark->lock);

	mark->mask = mask;

	if (mark->flags & FSNOTIFY_MARK_FLAG_INODE)
	fsnotify_set_inode_mark_mask_locked(mark, mask);
	}

	void fsnotify_set_mark_ignored_mask_locked(struct fsnotify_mark *mark, __u32 mask)
	{
	assert_spin_locked(&mark->lock);

	mark->ignored_mask = mask;
	}

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

	BUG_ON(inode && mnt);
	BUG_ON(!inode && !mnt);

	/*
	* if this group isn't being testing for inode type events we need
	* to start testing
	*/
	if (inode && unlikely(list_empty(&group->inode_group_list)))
	fsnotify_add_inode_group(group);
	else if (mnt && unlikely(list_empty(&group->vfsmount_group_list)))
	fsnotify_add_vfsmount_group(group);

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

	mark->flags \|= FSNOTIFY_MARK_FLAG_ALIVE;

	mark->group = group;
	list_add(&mark->g_list, &group->marks_list);
	atomic_inc(&group->num_marks);
	fsnotify_get_mark(mark); /* for i_list and g_list */

	if (inode) {
	ret = fsnotify_add_inode_mark(mark, group, inode, allow_dups);
	if (ret)
	goto err;
	} else if (mnt) {
	ret = fsnotify_add_vfsmount_mark(mark, group, mnt, allow_dups);
	if (ret)
	goto err;
	} else {
	BUG();
	}

	spin_unlock(&group->mark_lock);

	/* this will pin the object if appropriate */
	fsnotify_set_mark_mask_locked(mark, mark->mask);

	spin_unlock(&mark->lock);

	if (inode)
	__fsnotify_update_child_dentry_flags(inode);

	return ret;
	err:
	mark->flags &= ~FSNOTIFY_MARK_FLAG_ALIVE;
	list_del_init(&mark->g_list);
	mark->group = NULL;
	atomic_dec(&group->num_marks);

	spin_unlock(&group->mark_lock);
	spin_unlock(&mark->lock);

	spin_lock(&destroy_lock);
	list_add(&mark->destroy_list, &destroy_list);
	spin_unlock(&destroy_lock);
	wake_up(&destroy_waitq);

	return ret;
	}

	/*
	* clear any marks in a group in which mark->flags & flags is true
	*/
	void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
	unsigned int flags)
	{
	struct fsnotify_mark lmark, mark;
	LIST_HEAD(free_list);

	spin_lock(&group->mark_lock);
	list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
	if (mark->flags & flags) {
	list_add(&mark->free_g_list, &free_list);
	list_del_init(&mark->g_list);
	fsnotify_get_mark(mark);
	}
	}
	spin_unlock(&group->mark_lock);

	list_for_each_entry_safe(mark, lmark, &free_list, free_g_list) {
	fsnotify_destroy_mark(mark);
	fsnotify_put_mark(mark);
	}
	}

	/*
	* Given a group, destroy all of the marks associated with that group.
	*/
	void fsnotify_clear_marks_by_group(struct fsnotify_group *group)
	{
	fsnotify_clear_marks_by_group_flags(group, (unsigned int)-1);
	}

	void fsnotify_duplicate_mark(struct fsnotify_mark new, struct fsnotify_mark old)
	{
	assert_spin_locked(&old->lock);
	new->i.inode = old->i.inode;
	new->m.mnt = old->m.mnt;
	new->group = old->group;
	new->mask = old->mask;
	new->free_mark = old->free_mark;
	}

	/*
	* Nothing fancy, just initialize lists and locks and counters.
	*/
	void fsnotify_init_mark(struct fsnotify_mark *mark,
	void (free_mark)(struct fsnotify_mark mark))
	{
	memset(mark, 0, sizeof(*mark));
	spin_lock_init(&mark->lock);
	atomic_set(&mark->refcnt, 1);
	mark->free_mark = free_mark;
	}

	static int fsnotify_mark_destroy(void *ignored)
	{
	struct fsnotify_mark mark, next;
	LIST_HEAD(private_destroy_list);

	for (;;) {
	spin_lock(&destroy_lock);
	/* exchange the list head */
	list_replace_init(&destroy_list, &private_destroy_list);
	spin_unlock(&destroy_lock);

	synchronize_srcu(&fsnotify_mark_srcu);

	list_for_each_entry_safe(mark, next, &private_destroy_list, destroy_list) {
	list_del_init(&mark->destroy_list);
	fsnotify_put_mark(mark);
	}

	wait_event_interruptible(destroy_waitq, !list_empty(&destroy_list));
	}

	return 0;
	}

	static int __init fsnotify_mark_init(void)
	{
	struct task_struct *thread;

	thread = kthread_run(fsnotify_mark_destroy, NULL,
	"fsnotify_mark");
	if (IS_ERR(thread))
	panic("unable to start fsnotify mark destruction thread.");

	return 0;
	}
	device_initcall(fsnotify_mark_init);