On Wed, Jul 13, 2011 at 01:56:34AM +0100, Al Viro wrote: > Nick, could you please describe the locking rules you had in mind for > ->d_lock? unlazy_walk() (aka nameidata_dentry_drop_rcu()) can probably > be dealt with by checking d_seq twice, once before locking the child. > Then we could be sure that it's still a child of parent and will stay > so as long as parent's ->d_lock is held, and thus the ordering would > stay stable... As the matter of fact, can we ever get there with IS_ROOT(dentry)? AFAICS, that should be impossible - dentry->d_seq would have to be changed by whatever had torn it off the tree and we would have buggered off on __d_rcu_to_refcount() failing... AFAICS, the only way to get there would be with mountpoint crossing returning a symlink with symlink already killed by rename() somehow (call in walk_component()). The first part should be impossible - symlinks can't be mounted/bound on anything (and if it would be possible, we'd trigger that BUG_ON() if symlink was still alive, anyway). So here's what I want to do to unlazy_walk(); it'll almost certainly leave other problems with ->d_lock, but at least it'll take care of that one: Make sure that child is still a child of parent before nested locking of child->d_lock in unlazy_walk(); otherwise we are risking a violation of locking order and deadlocks. Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> --- diff --git a/fs/namei.c b/fs/namei.c index 0223c41..5c867dd 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -433,6 +433,8 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry) goto err_parent; BUG_ON(nd->inode != parent->d_inode); } else { + if (dentry->d_parent != parent) + goto err_parent; spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED); if (!__d_rcu_to_refcount(dentry, nd->seq)) goto err_child; -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html