linux-kernel - [PATCH 2/2] fsnotify: allow sleepable child flag update

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Date:   Mon, 17 Oct 2022 21:12:33 -0700
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>
Cc:     Amir Goldstein <amir73il@...il.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Stephen Brennan <stephen.s.brennan@...cle.com>
Subject: [PATCH 2/2] fsnotify: allow sleepable child flag update

With very large d_subdirs lists, iteration can take a long time. Since
iteration needs to hold parent->d_lock, this can trigger soft lockups.
It would be best to make this iteration sleepable. Since we have the
inode locked exclusive, we can drop the parent->d_lock and sleep,
holding a reference to a child dentry, and continue iteration once we
wake.

Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
---
 fs/notify/fsnotify.c | 72 ++++++++++++++++++++++++++++++--------------
 1 file changed, 50 insertions(+), 22 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index e887a195983b..499b19272b32 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -102,10 +102,13 @@ void fsnotify_sb_delete(struct super_block *sb)
  * on a child we run all of our children and set a dentry flag saying that the
  * 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.
+ *
+ * Context: inode locked exclusive
  */
 void __fsnotify_update_child_dentry_flags(struct inode *inode)
 {
-	struct dentry *alias;
+	struct dentry *child, *alias, *last_ref = NULL;
+	struct list_head *p;
 	int watched;
 
 	if (!S_ISDIR(inode->i_mode))
@@ -114,30 +117,55 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
 	/* 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 */
-	hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
-		struct dentry *child;
-
-		/* run all of the children of the original inode and fix their
-		 * d_flags to indicate parental interest (their parent is the
-		 * original inode) */
-		spin_lock(&alias->d_lock);
-		list_for_each_entry(child, &alias->d_subdirs, d_child) {
-			if (!child->d_inode)
-				continue;
+	alias = d_find_any_alias(inode);
+
+	/*
+	 * These lists can get very long, so we may need to sleep during
+	 * iteration. Normally this would be impossible without a cursor,
+	 * but since we have the inode locked exclusive, we're guaranteed
+	 * that the directory won't be modified, so whichever dentry we
+	 * pick to sleep on won't get moved. So, start a manual iteration
+	 * over d_subdirs which will allow us to sleep.
+	 */
+	spin_lock(&alias->d_lock);
+	p = alias->d_subdirs.next;
 
-			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;
-			spin_unlock(&child->d_lock);
+	while (p != &alias->d_subdirs) {
+		child = list_entry(p, struct dentry, d_child);
+		if (need_resched()) {
+			/*
+			 * We need to hold a reference while we sleep. But when
+			 * we wake, dput() could free the dentry, invalidating
+			 * the list pointers. We can't look at the list pointers
+			 * until we re-lock the parent, and we can't dput() once
+			 * we have the parent locked. So the solution is to hold
+			 * onto our reference and free it the *next* time we drop
+			 * alias->d_lock: either at the end of the function, or
+			 * at the time of the next sleep.
+			 */
+			dget(child);
+			spin_unlock(&alias->d_lock);
+			if (last_ref)
+				dput(last_ref);
+			last_ref = child;
+			cond_resched();
+			spin_lock(&alias->d_lock);
 		}
-		spin_unlock(&alias->d_lock);
+		p = p->next;
+
+		if (!child->d_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;
+		spin_unlock(&child->d_lock);
 	}
-	spin_unlock(&inode->i_lock);
+	spin_unlock(&alias->d_lock);
+	if (last_ref)
+		dput(last_ref);
 }
 
 /* Are inode/sb/mount interested in parent and name info with this event? */
-- 
2.34.1