Merge branch 'for-linus-4.13-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git...

[mirror_ubuntu-artful-kernel.git] / fs / notify / 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 group->recnt and mark->refcnt tell how many "things" in the kernel
 * currently are referencing the objects. Both kind of objects typically will
 * live inside the kernel with a refcnt of 2, one for its creation and one for
 * the reference a group and a mark hold to each other.
 * If you are holding the appropriate locks, you can take a reference and the
 * object itself is guaranteed to survive until the reference is dropped.
 *
 * LOCKING:
 * There are 3 locks involved with fsnotify inode marks and they MUST be taken
 * in order as follows:
 *
 * group->mark_mutex
 * mark->lock
 * mark->connector->lock
 *
 * group->mark_mutex protects the marks_list anchored inside a given group and
 * each mark is hooked via the g_list.  It also protects the groups private
 * data (i.e group limits).

 * mark->lock protects the marks attributes like its masks and flags.
 * Furthermore it protects the access to a reference of the group that the mark
 * is assigned to as well as the access to a reference of the inode/vfsmount
 * that is being watched by the mark.
 *
 * mark->connector->lock protects the list of marks anchored inside an
 * inode / vfsmount and each mark is hooked via the i_list.
 *
 * A list of notification marks relating to inode / mnt is contained in
 * fsnotify_mark_connector. That structure is alive as long as there are any
 * marks in the list and is also protected by fsnotify_mark_srcu. A mark gets
 * detached from fsnotify_mark_connector when last reference to the mark is
 * dropped.  Thus having mark reference is enough to protect mark->connector
 * pointer and to make sure fsnotify_mark_connector cannot disappear. Also
 * because we remove mark from g_list before dropping mark reference associated
 * with that, any mark found through g_list is guaranteed to have
 * mark->connector set until we drop group->mark_mutex.
 *
 * LIFETIME:
 * Inode marks survive between when they are added to an inode and when their
 * refcnt==0. Marks are also protected by fsnotify_mark_srcu.
 *
 * 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)
 *
 * 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/atomic.h>

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

#define FSNOTIFY_REAPER_DELAY   (1)     /* 1 jiffy */

struct srcu_struct fsnotify_mark_srcu;
struct kmem_cache *fsnotify_mark_connector_cachep;

static DEFINE_SPINLOCK(destroy_lock);
static LIST_HEAD(destroy_list);
static struct fsnotify_mark_connector *connector_destroy_list;

static void fsnotify_mark_destroy_workfn(struct work_struct *work);
static DECLARE_DELAYED_WORK(reaper_work, fsnotify_mark_destroy_workfn);

static void fsnotify_connector_destroy_workfn(struct work_struct *work);
static DECLARE_WORK(connector_reaper_work, fsnotify_connector_destroy_workfn);

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

/*
 * Get mark reference when we found the mark via lockless traversal of object
 * list. Mark can be already removed from the list by now and on its way to be
 * destroyed once SRCU period ends.
 */
static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
{
        return atomic_inc_not_zero(&mark->refcnt);
}

static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
        u32 new_mask = 0;
        struct fsnotify_mark *mark;

        assert_spin_locked(&conn->lock);
        hlist_for_each_entry(mark, &conn->list, obj_list) {
                if (mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)
                        new_mask |= mark->mask;
        }
        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
                conn->inode->i_fsnotify_mask = new_mask;
        else if (conn->flags & FSNOTIFY_OBJ_TYPE_VFSMOUNT)
                real_mount(conn->mnt)->mnt_fsnotify_mask = new_mask;
}

/*
 * Calculate mask of events for a list of marks. The caller must make sure
 * connector and connector->inode cannot disappear under us.  Callers achieve
 * this by holding a mark->lock or mark->group->mark_mutex for a mark on this
 * list.
 */
void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
        if (!conn)
                return;

        spin_lock(&conn->lock);
        __fsnotify_recalc_mask(conn);
        spin_unlock(&conn->lock);
        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE)
                __fsnotify_update_child_dentry_flags(conn->inode);
}

/* Free all connectors queued for freeing once SRCU period ends */
static void fsnotify_connector_destroy_workfn(struct work_struct *work)
{
        struct fsnotify_mark_connector *conn, *free;

        spin_lock(&destroy_lock);
        conn = connector_destroy_list;
        connector_destroy_list = NULL;
        spin_unlock(&destroy_lock);

        synchronize_srcu(&fsnotify_mark_srcu);
        while (conn) {
                free = conn;
                conn = conn->destroy_next;
                kmem_cache_free(fsnotify_mark_connector_cachep, free);
        }
}

static struct inode *fsnotify_detach_connector_from_object(
                                        struct fsnotify_mark_connector *conn)
{
        struct inode *inode = NULL;

        if (conn->flags & FSNOTIFY_OBJ_TYPE_INODE) {
                inode = conn->inode;
                rcu_assign_pointer(inode->i_fsnotify_marks, NULL);
                inode->i_fsnotify_mask = 0;
                conn->inode = NULL;
                conn->flags &= ~FSNOTIFY_OBJ_TYPE_INODE;
        } else if (conn->flags & FSNOTIFY_OBJ_TYPE_VFSMOUNT) {
                rcu_assign_pointer(real_mount(conn->mnt)->mnt_fsnotify_marks,
                                   NULL);
                real_mount(conn->mnt)->mnt_fsnotify_mask = 0;
                conn->mnt = NULL;
                conn->flags &= ~FSNOTIFY_OBJ_TYPE_VFSMOUNT;
        }

        return inode;
}

static void fsnotify_final_mark_destroy(struct fsnotify_mark *mark)
{
        struct fsnotify_group *group = mark->group;

        if (WARN_ON_ONCE(!group))
                return;
        group->ops->free_mark(mark);
        fsnotify_put_group(group);
}

void fsnotify_put_mark(struct fsnotify_mark *mark)
{
        struct fsnotify_mark_connector *conn;
        struct inode *inode = NULL;
        bool free_conn = false;

        /* Catch marks that were actually never attached to object */
        if (!mark->connector) {
                if (atomic_dec_and_test(&mark->refcnt))
                        fsnotify_final_mark_destroy(mark);
                return;
        }

        /*
         * We have to be careful so that traversals of obj_list under lock can
         * safely grab mark reference.
         */
        if (!atomic_dec_and_lock(&mark->refcnt, &mark->connector->lock))
                return;

        conn = mark->connector;
        hlist_del_init_rcu(&mark->obj_list);
        if (hlist_empty(&conn->list)) {
                inode = fsnotify_detach_connector_from_object(conn);
                free_conn = true;
        } else {
                __fsnotify_recalc_mask(conn);
        }
        mark->connector = NULL;
        spin_unlock(&conn->lock);

        iput(inode);

        if (free_conn) {
                spin_lock(&destroy_lock);
                conn->destroy_next = connector_destroy_list;
                connector_destroy_list = conn;
                spin_unlock(&destroy_lock);
                queue_work(system_unbound_wq, &connector_reaper_work);
        }
        /*
         * Note that we didn't update flags telling whether inode cares about
         * what's happening with children. We update these flags from
         * __fsnotify_parent() lazily when next event happens on one of our
         * children.
         */
        spin_lock(&destroy_lock);
        list_add(&mark->g_list, &destroy_list);
        spin_unlock(&destroy_lock);
        queue_delayed_work(system_unbound_wq, &reaper_work,
                           FSNOTIFY_REAPER_DELAY);
}

bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
{
        struct fsnotify_group *group;

        if (WARN_ON_ONCE(!iter_info->inode_mark && !iter_info->vfsmount_mark))
                return false;

        if (iter_info->inode_mark)
                group = iter_info->inode_mark->group;
        else
                group = iter_info->vfsmount_mark->group;

        /*
         * Since acquisition of mark reference is an atomic op as well, we can
         * be sure this inc is seen before any effect of refcount increment.
         */
        atomic_inc(&group->user_waits);

        if (iter_info->inode_mark) {
                /* This can fail if mark is being removed */
                if (!fsnotify_get_mark_safe(iter_info->inode_mark))
                        goto out_wait;
        }
        if (iter_info->vfsmount_mark) {
                if (!fsnotify_get_mark_safe(iter_info->vfsmount_mark))
                        goto out_inode;
        }

        /*
         * Now that both marks are pinned by refcount in the inode / vfsmount
         * lists, we can drop SRCU lock, and safely resume the list iteration
         * once userspace returns.
         */
        srcu_read_unlock(&fsnotify_mark_srcu, iter_info->srcu_idx);

        return true;
out_inode:
        if (iter_info->inode_mark)
                fsnotify_put_mark(iter_info->inode_mark);
out_wait:
        if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
                wake_up(&group->notification_waitq);
        return false;
}

void fsnotify_finish_user_wait(struct fsnotify_iter_info *iter_info)
{
        struct fsnotify_group *group = NULL;

        iter_info->srcu_idx = srcu_read_lock(&fsnotify_mark_srcu);
        if (iter_info->inode_mark) {
                group = iter_info->inode_mark->group;
                fsnotify_put_mark(iter_info->inode_mark);
        }
        if (iter_info->vfsmount_mark) {
                group = iter_info->vfsmount_mark->group;
                fsnotify_put_mark(iter_info->vfsmount_mark);
        }
        /*
         * We abuse notification_waitq on group shutdown for waiting for all
         * marks pinned when waiting for userspace.
         */
        if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
                wake_up(&group->notification_waitq);
}

/*
 * Mark mark as detached, remove it from group list. Mark still stays in object
 * list until its last reference is dropped. Note that we rely on mark being
 * removed from group list before corresponding reference to it is dropped. In
 * particular we rely on mark->connector being valid while we hold
 * group->mark_mutex if we found the mark through g_list.
 *
 * Must be called with group->mark_mutex held. The caller must either hold
 * reference to the mark or be protected by fsnotify_mark_srcu.
 */
void fsnotify_detach_mark(struct fsnotify_mark *mark)
{
        struct fsnotify_group *group = mark->group;

        WARN_ON_ONCE(!mutex_is_locked(&group->mark_mutex));
        WARN_ON_ONCE(!srcu_read_lock_held(&fsnotify_mark_srcu) &&
                     atomic_read(&mark->refcnt) < 1 +
                        !!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED));

        spin_lock(&mark->lock);
        /* something else already called this function on this mark */
        if (!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
                spin_unlock(&mark->lock);
                return;
        }
        mark->flags &= ~FSNOTIFY_MARK_FLAG_ATTACHED;
        list_del_init(&mark->g_list);
        spin_unlock(&mark->lock);

        atomic_dec(&group->num_marks);

        /* Drop mark reference acquired in fsnotify_add_mark_locked() */
        fsnotify_put_mark(mark);
}

/*
 * Free fsnotify mark. The mark is actually only marked as being freed.  The
 * freeing is actually happening only once last reference to the mark is
 * dropped from a workqueue which first waits for srcu period end.
 *
 * Caller must have a reference to the mark or be protected by
 * fsnotify_mark_srcu.
 */
void fsnotify_free_mark(struct fsnotify_mark *mark)
{
        struct fsnotify_group *group = mark->group;

        spin_lock(&mark->lock);
        /* 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;
        spin_unlock(&mark->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 mark
         * is being freed.
         */
        if (group->ops->freeing_mark)
                group->ops->freeing_mark(mark, group);
}

void fsnotify_destroy_mark(struct fsnotify_mark *mark,
                           struct fsnotify_group *group)
{
        mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
        fsnotify_detach_mark(mark);
        mutex_unlock(&group->mark_mutex);
        fsnotify_free_mark(mark);
}

/*
 * Sorting function for lists of fsnotify marks.
 *
 * Fanotify supports different notification classes (reflected as priority of
 * notification group). Events shall be passed to notification groups in
 * decreasing priority order. To achieve this marks in notification lists for
 * inodes and vfsmounts are sorted so that priorities of corresponding groups
 * are descending.
 *
 * Furthermore correct handling of the ignore mask requires processing inode
 * and vfsmount marks of each group together. Using the group address as
 * further sort criterion provides a unique sorting order and thus we can
 * merge inode and vfsmount lists of marks in linear time and find groups
 * present in both lists.
 *
 * A return value of 1 signifies that b has priority over a.
 * A return value of 0 signifies that the two marks have to be handled together.
 * A return value of -1 signifies that a has priority over b.
 */
int fsnotify_compare_groups(struct fsnotify_group *a, struct fsnotify_group *b)
{
        if (a == b)
                return 0;
        if (!a)
                return 1;
        if (!b)
                return -1;
        if (a->priority < b->priority)
                return 1;
        if (a->priority > b->priority)
                return -1;
        if (a < b)
                return 1;
        return -1;
}

static int fsnotify_attach_connector_to_object(
                                struct fsnotify_mark_connector __rcu **connp,
                                struct inode *inode,
                                struct vfsmount *mnt)
{
        struct fsnotify_mark_connector *conn;

        conn = kmem_cache_alloc(fsnotify_mark_connector_cachep, GFP_KERNEL);
        if (!conn)
                return -ENOMEM;
        spin_lock_init(&conn->lock);
        INIT_HLIST_HEAD(&conn->list);
        if (inode) {
                conn->flags = FSNOTIFY_OBJ_TYPE_INODE;
                conn->inode = igrab(inode);
        } else {
                conn->flags = FSNOTIFY_OBJ_TYPE_VFSMOUNT;
                conn->mnt = mnt;
        }
        /*
         * cmpxchg() provides the barrier so that readers of *connp can see
         * only initialized structure
         */
        if (cmpxchg(connp, NULL, conn)) {
                /* Someone else created list structure for us */
                if (inode)
                        iput(inode);
                kmem_cache_free(fsnotify_mark_connector_cachep, conn);
        }

        return 0;
}

/*
 * Get mark connector, make sure it is alive and return with its lock held.
 * This is for users that get connector pointer from inode or mount. Users that
 * hold reference to a mark on the list may directly lock connector->lock as
 * they are sure list cannot go away under them.
 */
static struct fsnotify_mark_connector *fsnotify_grab_connector(
                                struct fsnotify_mark_connector __rcu **connp)
{
        struct fsnotify_mark_connector *conn;
        int idx;

        idx = srcu_read_lock(&fsnotify_mark_srcu);
        conn = srcu_dereference(*connp, &fsnotify_mark_srcu);
        if (!conn)
                goto out;
        spin_lock(&conn->lock);
        if (!(conn->flags & (FSNOTIFY_OBJ_TYPE_INODE |
                             FSNOTIFY_OBJ_TYPE_VFSMOUNT))) {
                spin_unlock(&conn->lock);
                srcu_read_unlock(&fsnotify_mark_srcu, idx);
                return NULL;
        }
out:
        srcu_read_unlock(&fsnotify_mark_srcu, idx);
        return conn;
}

/*
 * Add mark into proper place in given list of marks. These marks may be used
 * for the fsnotify backend to determine which event types should be delivered
 * to which group and for which inodes. These marks are ordered according to
 * priority, highest number first, and then by the group's location in memory.
 */
static int fsnotify_add_mark_list(struct fsnotify_mark *mark,
                                  struct inode *inode, struct vfsmount *mnt,
                                  int allow_dups)
{
        struct fsnotify_mark *lmark, *last = NULL;
        struct fsnotify_mark_connector *conn;
        struct fsnotify_mark_connector __rcu **connp;
        int cmp;
        int err = 0;

        if (WARN_ON(!inode && !mnt))
                return -EINVAL;
        if (inode)
                connp = &inode->i_fsnotify_marks;
        else
                connp = &real_mount(mnt)->mnt_fsnotify_marks;
restart:
        spin_lock(&mark->lock);
        conn = fsnotify_grab_connector(connp);
        if (!conn) {
                spin_unlock(&mark->lock);
                err = fsnotify_attach_connector_to_object(connp, inode, mnt);
                if (err)
                        return err;
                goto restart;
        }

        /* is mark the first mark? */
        if (hlist_empty(&conn->list)) {
                hlist_add_head_rcu(&mark->obj_list, &conn->list);
                goto added;
        }

        /* should mark be in the middle of the current list? */
        hlist_for_each_entry(lmark, &conn->list, obj_list) {
                last = lmark;

                if ((lmark->group == mark->group) &&
                    (lmark->flags & FSNOTIFY_MARK_FLAG_ATTACHED) &&
                    !allow_dups) {
                        err = -EEXIST;
                        goto out_err;
                }

                cmp = fsnotify_compare_groups(lmark->group, mark->group);
                if (cmp >= 0) {
                        hlist_add_before_rcu(&mark->obj_list, &lmark->obj_list);
                        goto added;
                }
        }

        BUG_ON(last == NULL);
        /* mark should be the last entry.  last is the current last entry */
        hlist_add_behind_rcu(&mark->obj_list, &last->obj_list);
added:
        mark->connector = conn;
out_err:
        spin_unlock(&conn->lock);
        spin_unlock(&mark->lock);
        return err;
}

/*
 * 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_locked(struct fsnotify_mark *mark, struct inode *inode,
                             struct vfsmount *mnt, int allow_dups)
{
        struct fsnotify_group *group = mark->group;
        int ret = 0;

        BUG_ON(inode && mnt);
        BUG_ON(!inode && !mnt);
        BUG_ON(!mutex_is_locked(&group->mark_mutex));

        /*
         * LOCKING ORDER!!!!
         * group->mark_mutex
         * mark->lock
         * mark->connector->lock
         */
        spin_lock(&mark->lock);
        mark->flags |= FSNOTIFY_MARK_FLAG_ALIVE | FSNOTIFY_MARK_FLAG_ATTACHED;

        list_add(&mark->g_list, &group->marks_list);
        atomic_inc(&group->num_marks);
        fsnotify_get_mark(mark); /* for g_list */
        spin_unlock(&mark->lock);

        ret = fsnotify_add_mark_list(mark, inode, mnt, allow_dups);
        if (ret)
                goto err;

        if (mark->mask)
                fsnotify_recalc_mask(mark->connector);

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

        fsnotify_put_mark(mark);
        return ret;
}

int fsnotify_add_mark(struct fsnotify_mark *mark, struct inode *inode,
                      struct vfsmount *mnt, int allow_dups)
{
        int ret;
        struct fsnotify_group *group = mark->group;

        mutex_lock(&group->mark_mutex);
        ret = fsnotify_add_mark_locked(mark, inode, mnt, allow_dups);
        mutex_unlock(&group->mark_mutex);
        return ret;
}

/*
 * Given a list of marks, find the mark associated with given group. If found
 * take a reference to that mark and return it, else return NULL.
 */
struct fsnotify_mark *fsnotify_find_mark(
                                struct fsnotify_mark_connector __rcu **connp,
                                struct fsnotify_group *group)
{
        struct fsnotify_mark_connector *conn;
        struct fsnotify_mark *mark;

        conn = fsnotify_grab_connector(connp);
        if (!conn)
                return NULL;

        hlist_for_each_entry(mark, &conn->list, obj_list) {
                if (mark->group == group &&
                    (mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
                        fsnotify_get_mark(mark);
                        spin_unlock(&conn->lock);
                        return mark;
                }
        }
        spin_unlock(&conn->lock);
        return NULL;
}

/* Clear any marks in a group with given type */
void fsnotify_clear_marks_by_group(struct fsnotify_group *group,
                                   unsigned int type)
{
        struct fsnotify_mark *lmark, *mark;
        LIST_HEAD(to_free);
        struct list_head *head = &to_free;

        /* Skip selection step if we want to clear all marks. */
        if (type == FSNOTIFY_OBJ_ALL_TYPES) {
                head = &group->marks_list;
                goto clear;
        }
        /*
         * We have to be really careful here. Anytime we drop mark_mutex, e.g.
         * fsnotify_clear_marks_by_inode() can come and free marks. Even in our
         * to_free list so we have to use mark_mutex even when accessing that
         * list. And freeing mark requires us to drop mark_mutex. So we can
         * reliably free only the first mark in the list. That's why we first
         * move marks to free to to_free list in one go and then free marks in
         * to_free list one by one.
         */
        mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
        list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
                if (mark->connector->flags & type)
                        list_move(&mark->g_list, &to_free);
        }
        mutex_unlock(&group->mark_mutex);

clear:
        while (1) {
                mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
                if (list_empty(head)) {
                        mutex_unlock(&group->mark_mutex);
                        break;
                }
                mark = list_first_entry(head, struct fsnotify_mark, g_list);
                fsnotify_get_mark(mark);
                fsnotify_detach_mark(mark);
                mutex_unlock(&group->mark_mutex);
                fsnotify_free_mark(mark);
                fsnotify_put_mark(mark);
        }
}

/* Destroy all marks attached to inode / vfsmount */
void fsnotify_destroy_marks(struct fsnotify_mark_connector __rcu **connp)
{
        struct fsnotify_mark_connector *conn;
        struct fsnotify_mark *mark, *old_mark = NULL;
        struct inode *inode;

        conn = fsnotify_grab_connector(connp);
        if (!conn)
                return;
        /*
         * We have to be careful since we can race with e.g.
         * fsnotify_clear_marks_by_group() and once we drop the conn->lock, the
         * list can get modified. However we are holding mark reference and
         * thus our mark cannot be removed from obj_list so we can continue
         * iteration after regaining conn->lock.
         */
        hlist_for_each_entry(mark, &conn->list, obj_list) {
                fsnotify_get_mark(mark);
                spin_unlock(&conn->lock);
                if (old_mark)
                        fsnotify_put_mark(old_mark);
                old_mark = mark;
                fsnotify_destroy_mark(mark, mark->group);
                spin_lock(&conn->lock);
        }
        /*
         * Detach list from object now so that we don't pin inode until all
         * mark references get dropped. It would lead to strange results such
         * as delaying inode deletion or blocking unmount.
         */
        inode = fsnotify_detach_connector_from_object(conn);
        spin_unlock(&conn->lock);
        if (old_mark)
                fsnotify_put_mark(old_mark);
        iput(inode);
}

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

/*
 * Destroy all marks in destroy_list, waits for SRCU period to finish before
 * actually freeing marks.
 */
static void fsnotify_mark_destroy_workfn(struct work_struct *work)
{
        struct fsnotify_mark *mark, *next;
        struct list_head private_destroy_list;

        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, g_list) {
                list_del_init(&mark->g_list);
                fsnotify_final_mark_destroy(mark);
        }
}

/* Wait for all marks queued for destruction to be actually destroyed */
void fsnotify_wait_marks_destroyed(void)
{
        flush_delayed_work(&reaper_work);
}