[PATCH v2 07/18] fsnotify: reduce i_lock hold time in fsnotify_set_children_dentry_flags()

From: NeilBrown <neil-+NVA1uvv1dVBDLzU/[email protected]>

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-+NVA1uvv1dVBDLzU/[email protected]>
---
 fs/notify/fsnotify.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 90a2121fc54a..4a383d4d8a7b 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -71,27 +71,28 @@ 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) */
-		d_for_each_positive_child(child, alias) {
-			spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
-			child->d_flags |= DCACHE_FSNOTIFY_PARENT_WATCHED;
-			spin_unlock(&child->d_lock);
-		}
+	/* Find the dentry for inode - there can only be one */
+	alias = d_find_alias(inode);
+
+	if (!alias)
+		return;
+
+	/*
+	 * run all of the children of the original inode and fix their
+	 * d_flags to indicate parental interest (their parent is the
+	 * original inode)
+	 */
+	d_for_each_positive_child(child, alias) {
+		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);
+	dput(alias);
 }
 
 /*
-- 
2.50.0.107.gf914562f5916.dirty