Commit c2338f2d authored Jun 12, 2014 by Al Viro

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>

parent 9c1d5284

fs/dcache.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -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);

Admin message