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


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