| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <20250903093413.3434-2-lidiangang@bytedance.com>
Date: Wed, 3 Sep 2025 17:34:13 +0800
From: Diangang Li <lidiangang@...edance.com>
To: jack@...e.cz,
amir73il@...il.com,
stephen.s.brennan@...cle.com,
changfengnan@...edance.com
Cc: linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org,
Sasha Levin <sashal@...nel.org>,
Diangang Li <lidiangang@...edance.com>
Subject: [RFC 1/1] fsnotify: clear PARENT_WATCHED flags lazily
From: Amir Goldstein <amir73il@...il.com>
commit 172e422ffea2 ("fsnotify: clear PARENT_WATCHED flags lazily")
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@...e.cz>
Signed-off-by: Amir Goldstein <amir73il@...il.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Sasha Levin <sashal@...nel.org>
Signed-off-by: Diangang Li <lidiangang@...edance.com>
---
fs/notify/fsnotify.c | 31 +++++++++++++++++++++----------
fs/notify/fsnotify.h | 2 +-
fs/notify/mark.c | 32 +++++++++++++++++++++++++++++---
include/linux/fsnotify_backend.h | 8 +++++---
4 files changed, 56 insertions(+), 17 deletions(-)
diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index f44e39c68328..d5757fd69f62 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -105,17 +105,13 @@ void fsnotify_sb_delete(struct super_block *sb)
* parent cares. Thus when an event happens on a child it can quickly tell if
* 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 */
@@ -131,10 +127,7 @@ 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);
@@ -142,6 +135,24 @@ 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);
+}
+
/* Notify this dentry's parent about a child's events. */
int __fsnotify_parent(const struct path *path, struct dentry *dentry, __u32 mask)
{
@@ -159,7 +170,7 @@ int __fsnotify_parent(const struct path *path, struct dentry *dentry, __u32 mask
p_inode = parent->d_inode;
if (unlikely(!fsnotify_inode_watches_children(p_inode))) {
- __fsnotify_update_child_dentry_flags(p_inode);
+ fsnotify_clear_child_dentry_flag(p_inode, dentry);
} else if (p_inode->i_fsnotify_mask & mask & ALL_FSNOTIFY_EVENTS) {
struct name_snapshot name;
diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
index f3462828a0e2..0ac1f8e8e012 100644
--- a/fs/notify/fsnotify.h
+++ b/fs/notify/fsnotify.h
@@ -59,7 +59,7 @@ 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);
/* allocate and destroy and event holder to attach events to notification/access queues */
extern struct fsnotify_event_holder *fsnotify_alloc_event_holder(void);
diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index fdf8e03bf3df..ba4953b0917d 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -132,6 +132,24 @@ static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
*fsnotify_conn_mask_p(conn) = new_mask;
}
+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
@@ -140,15 +158,23 @@ 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 */
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 64cfb5446f4d..d7ced985ffee 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -360,12 +360,14 @@ extern u32 fsnotify_get_cookie(void);
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;
}
/*
@@ -379,7 +381,7 @@ 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.
*/
--
2.39.5
Powered by blists - more mailing lists