From: Al Viro Date: Thu, 12 Jun 2014 04:29:13 +0000 (-0400) Subject: lock_parent: don't step on stale ->d_parent of all-but-freed one X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=c2338f2dc7c1e9f6202f370c64ffd7f44f3d4b51;p=openwrt%2Fstaging%2Fblogic.git 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 --- diff --git a/fs/dcache.c b/fs/dcache.c index be2bea834bf4..e99c6f529ba8 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -532,10 +532,12 @@ 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);