Re: [PATCH 1/2] vfs: dcache: fix deadlock in tree traversal

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Sep 17, 2012 at 10:23:30PM +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> IBM reported a deadlock in select_parent().  This was found to be caused by
> taking rename_lock when already locked when restarting the tree traversal.
> 
> There are two cases when the traversal needs to be restarted:
> 
>  1) concurrent d_move(); this can only happen when not already locked,
>  since taking rename_lock protects against concurrent d_move().
> 
>  2) racing with final d_put() on child just at the moment of ascending
>  to parent; rename_lock doesn't protect against this rare race, so it
>  can happen when already locked.
> 
> Because of case 2. we need to be able to handle restarting the traversal
> when rename_lock is already held.  This patch fixes all three callers of
> try_to_ascend().
> 
> IBM reported that the deadlock is gone with this patch.  However, there's still
> a soft lockup which is addressed by the next patch.

Egads...  The problem is real and analysis, AFAICS, is correct, but result
is extremely ugly ;-/  Let me try to come up with something saner; I'll
push that one to Linus if nothing better comes to mind, but I'd really
prefer to avoid adding to ugliness in fs/dcache.c - we already have too
much of that...