[patch added to the 3.12 stable tree] lock_parent: don't step on stale ->d_parent of all-but-freed one

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From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

This patch has been added to the 3.12 stable tree. If you have any
objections, please let us know.

===============

commit c2338f2dc7c1e9f6202f370c64ffd7f44f3d4b51 upstream.

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 fs/dcache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 85bc01f4bbca..e15f90c0e96a 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -554,10 +554,12 @@ 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);
-- 
2.1.2

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