lock_parent: don't step on stale ->d_parent of all-but-freed one
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@zeniv.linux.org.uk>
diff --git a/fs/dcache.c b/fs/dcache.c
index be2bea8..e99c6f5 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -532,10 +532,12 @@
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);