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fsnotify: clear PARENT_WATCHED flags lazily

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In some setups directories can have many (usually negative) dentries.
Hence __fsnotify_update_child_dentry_flags() function can take a
significant amount of time. Since the bulk of this function happens
under inode->i_lock this causes a significant contention on the lock
when we remove the watch from the directory as the
__fsnotify_update_child_dentry_flags() call from fsnotify_recalc_mask()
races with __fsnotify_update_child_dentry_flags() calls from
__fsnotify_parent() happening on children. This can lead upto softlockup
reports reported by users.

Fix the problem by calling fsnotify_update_children_dentry_flags() to
set PARENT_WATCHED flags only when parent starts watching children.

When parent stops watching children, clear false positive PARENT_WATCHED
flags lazily in __fsnotify_parent() for each accessed child.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Amir Goldstein 2024-05-12 13:30:07 +02:00 committed by Jan Kara
parent 32f88d65f0
commit 172e422ffe
4 changed files with 56 additions and 17 deletions
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31
fs/notify/fsnotify.c
View file

@ -117,17 +117,13 @@ void fsnotify_sb_free(struct super_block *sb)
* parent cares. Thus when an event happens on a child it can quickly tell
* if there is a need to find a parent and send the event to the parent.
*/
void __fsnotify_update_child_dentry_flags(struct inode *inode)
void fsnotify_set_children_dentry_flags(struct inode *inode)
{
struct dentry *alias;
int watched;
if (!S_ISDIR(inode->i_mode))
return;
/* determine if the children should tell inode about their events */
watched = fsnotify_inode_watches_children(inode);
spin_lock(&inode->i_lock);
/* run all of the dentries associated with this inode. Since this is a
* directory, there damn well better only be one item on this list */
@ -143,10 +139,7 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
continue;
spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
if (watched)
child->d_flags |= DCACHE_FSNOTIFY_PARENT_WATCHED;
else
child->d_flags &= ~DCACHE_FSNOTIFY_PARENT_WATCHED;
child->d_flags |= DCACHE_FSNOTIFY_PARENT_WATCHED;
spin_unlock(&child->d_lock);
}
spin_unlock(&alias->d_lock);
@ -154,6 +147,24 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
spin_unlock(&inode->i_lock);
}
/*
* Lazily clear false positive PARENT_WATCHED flag for child whose parent had
* stopped watching children.
*/
static void fsnotify_clear_child_dentry_flag(struct inode *pinode,
struct dentry *dentry)
{
spin_lock(&dentry->d_lock);
/*
* d_lock is a sufficient barrier to prevent observing a non-watched
* parent state from before the fsnotify_set_children_dentry_flags()
* or fsnotify_update_flags() call that had set PARENT_WATCHED.
*/
if (!fsnotify_inode_watches_children(pinode))
dentry->d_flags &= ~DCACHE_FSNOTIFY_PARENT_WATCHED;
spin_unlock(&dentry->d_lock);
}
/* Are inode/sb/mount interested in parent and name info with this event? */
static bool fsnotify_event_needs_parent(struct inode *inode, __u32 mnt_mask,
__u32 mask)
@ -228,7 +239,7 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
p_inode = parent->d_inode;
p_mask = fsnotify_inode_watches_children(p_inode);
if (unlikely(parent_watched && !p_mask))
__fsnotify_update_child_dentry_flags(p_inode);
fsnotify_clear_child_dentry_flag(p_inode, dentry);
/*
* Include parent/name in notification either if some notification

2
fs/notify/fsnotify.h
View file

@ -93,7 +93,7 @@ static inline void fsnotify_clear_marks_by_sb(struct super_block *sb)
* update the dentry->d_flags of all of inode's children to indicate if inode cares
* about events that happen to its children.
*/
extern void __fsnotify_update_child_dentry_flags(struct inode *inode);
extern void fsnotify_set_children_dentry_flags(struct inode *inode);
extern struct kmem_cache *fsnotify_mark_connector_cachep;

32
fs/notify/mark.c
View file

@ -250,6 +250,24 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
return fsnotify_update_iref(conn, want_iref);
}
static bool fsnotify_conn_watches_children(
struct fsnotify_mark_connector *conn)
{
if (conn->type != FSNOTIFY_OBJ_TYPE_INODE)
return false;
return fsnotify_inode_watches_children(fsnotify_conn_inode(conn));
}
static void fsnotify_conn_set_children_dentry_flags(
struct fsnotify_mark_connector *conn)
{
if (conn->type != FSNOTIFY_OBJ_TYPE_INODE)
return;
fsnotify_set_children_dentry_flags(fsnotify_conn_inode(conn));
}
/*
* Calculate mask of events for a list of marks. The caller must make sure
* connector and connector->obj cannot disappear under us. Callers achieve
@ -258,15 +276,23 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
*/
void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
{
bool update_children;
if (!conn)
return;
spin_lock(&conn->lock);
update_children = !fsnotify_conn_watches_children(conn);
__fsnotify_recalc_mask(conn);
update_children &= fsnotify_conn_watches_children(conn);
spin_unlock(&conn->lock);
if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
__fsnotify_update_child_dentry_flags(
fsnotify_conn_inode(conn));
/*
* Set children's PARENT_WATCHED flags only if parent started watching.
* When parent stops watching, we clear false positive PARENT_WATCHED
* flags lazily in __fsnotify_parent().
*/
if (update_children)
fsnotify_conn_set_children_dentry_flags(conn);
}
/* Free all connectors queued for freeing once SRCU period ends */

8
include/linux/fsnotify_backend.h
View file

@ -594,12 +594,14 @@ static inline __u32 fsnotify_parent_needed_mask(__u32 mask)
static inline int fsnotify_inode_watches_children(struct inode *inode)
{
__u32 parent_mask = READ_ONCE(inode->i_fsnotify_mask);
/* FS_EVENT_ON_CHILD is set if the inode may care */
if (!(inode->i_fsnotify_mask & FS_EVENT_ON_CHILD))
if (!(parent_mask & FS_EVENT_ON_CHILD))
return 0;
/* this inode might care about child events, does it care about the
* specific set of events that can happen on a child? */
return inode->i_fsnotify_mask & FS_EVENTS_POSS_ON_CHILD;
return parent_mask & FS_EVENTS_POSS_ON_CHILD;
}
/*
@ -613,7 +615,7 @@ static inline void fsnotify_update_flags(struct dentry *dentry)
/*
* Serialisation of setting PARENT_WATCHED on the dentries is provided
* by d_lock. If inotify_inode_watched changes after we have taken
* d_lock, the following __fsnotify_update_child_dentry_flags call will
* d_lock, the following fsnotify_set_children_dentry_flags call will
* find our entry, so it will spin until we complete here, and update
* us with the new state.
*/