[Devel] [PATCH RH7 2/2] ms/lock_parent: don't step on stale ->d_parent of all-but-freed one

[Vasily Averin]

From: Al Viro <viro at zeniv.linux.org.uk>

Dentry that had been through (or into) __dentry_kill() might be seen
by shrink_dentry_list(); that's normal, it'll be taken off the shrink
list and freed if __dentry_kill() has already finished.  The problem
is, its ->d_parent might be pointing to already freed dentry, so
lock_parent() needs to be careful.

We need to check that dentry hasn't already gone into __dentry_kill()
*and* grab rcu_read_lock() before dropping ->d_lock - the latter makes
sure that whatever we see in ->d_parent after dropping ->d_lock it
won't be freed until we drop rcu_read_lock().

Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
(cherry picked from commit c2338f2dc7c1e9f6202f370c64ffd7f44f3d4b51)
Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 fs/dcache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 41e8f55bd364..3d6b9b4623fd 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -701,10 +701,12 @@ static inline struct dentry *lock_parent(struct dentry *dentry)
 	struct dentry *parent = dentry->d_parent;
 	if (IS_ROOT(dentry))
 		return NULL;
+	if (unlikely((int)dentry->d_lockref.count < 0))
+		return NULL;
 	if (likely(spin_trylock(&parent->d_lock)))
 		return parent;
-	spin_unlock(&dentry->d_lock);
 	rcu_read_lock();
+	spin_unlock(&dentry->d_lock);
 again:
 	parent = ACCESS_ONCE(dentry->d_parent);
 	spin_lock(&parent->d_lock);
-- 
2.17.1