tree 54677cd1be2c934fea684a53e34f85d4c6f4981e
parent 9c1d5284c79fea050f115eadeec1dd1758e5c630
author Al Viro <viro@zeniv.linux.org.uk> 1402547353 -0400
committer Al Viro <viro@zeniv.linux.org.uk> 1402547353 -0400

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>