[PATCH v4 3/5] fsnotify: don't hold i_lock while walking d_children in fsnotify_set_children_dentry_flags()

[NeilBrown]

From: NeilBrown <neil@xxxxxxxxxx>

Rather than hold i_lock across a d_children walk, dget() a reference to
the dentry and drop the i_lock before the walk.  This is easily done
with d_find_alias().

This requires that we dput() which can sleep.  Now that no locks are
held across calls to fsnotify_set_children_dentry_flags(), this is safe.

Signed-off-by: NeilBrown <neil@xxxxxxxxxx>
---
 fs/notify/fsnotify.c | 40 ++++++++++++++++++++--------------------
 1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 7e2f330fd283..465e875dec57 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -71,32 +71,32 @@ void fsnotify_sb_free(struct super_block *sb)
  */
 void fsnotify_set_children_dentry_flags(struct inode *inode)
 {
-	struct dentry *alias;
+	struct dentry *alias, *child;
 
 	if (!S_ISDIR(inode->i_mode))
 		return;
 
-	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 */
-	for_each_alias(alias, inode) {
-		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);
-		hlist_for_each_entry(child, &alias->d_children, d_sib) {
-			if (!child->d_inode)
-				continue;
+	/* Find the dentry for inode - there can only be one */
+	alias = d_find_alias(inode);
+	if (!alias)
+		return;
 
-			spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
-			child->d_flags |= DCACHE_FSNOTIFY_PARENT_WATCHED;
-			spin_unlock(&child->d_lock);
-		}
-		spin_unlock(&alias->d_lock);
+	/*
+	 * 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);
+	hlist_for_each_entry(child, &alias->d_children, d_sib) {
+		if (!child->d_inode)
+			continue;
+
+		spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
+		child->d_flags |= DCACHE_FSNOTIFY_PARENT_WATCHED;
+		spin_unlock(&child->d_lock);
 	}
-	spin_unlock(&inode->i_lock);
+	spin_unlock(&alias->d_lock);
+	dput(alias);
 }
 
 /*
-- 
2.50.0.107.gf914562f5916.dirty