linux-kernel - Re: [PATCH] atomic: Fix _atomic_dec_and

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Date:	Mon, 15 Jun 2009 15:12:23 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	npiggin@...e.de, jblunck@...e.de, linux-kernel@...r.kernel.org,
	paulmck@...ibm.com
Subject: Re: [PATCH] atomic: Fix _atomic_dec_and_lock() deadlock on UP

On Mon, Jun 15, 2009 at 11:45:43AM -0700, Andrew Morton wrote:
> On Mon, 15 Jun 2009 14:11:13 -0400
> Valerie Aurora <vaurora@...hat.com> wrote:
> 
> > _atomic_dec_and_lock() can deadlock on UP with spinlock debugging
> > enabled.  Currently, on UP we unconditionally spin_lock() first, which
> > calls __spin_lock_debug(), which takes the lock unconditionally even
> > on UP.  This will deadlock in situations in which we call
> > atomic_dec_and_lock() knowing that the counter won't go to zero
> > (because we hold another reference) and that we already hold the lock.
> > Instead, we should use the SMP code path which only takes the lock if
> > necessary.
> 
> Yup, I have this queued for 2.6.31 as
> atomic-only-take-lock-when-the-counter-drops-to-zero-on-up-as-well.patch,
> with a different changelog:
> 
>   _atomic_dec_and_lock() should not unconditionally take the lock before
>   calling atomic_dec_and_test() in the UP case.  For consistency reasons it
>   should behave exactly like in the SMP case.
> 
>   Besides that this works around the problem that with CONFIG_DEBUG_SPINLOCK
>   this spins in __spin_lock_debug() if the lock is already taken even if the
>   counter doesn't drop to 0.
> 
>   Signed-off-by: Jan Blunck <jblunck@...e.de>
>   Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>   Acked-by: Nick Piggin <npiggin@...e.de>
>   Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> 
> 
> I can't remember why we decided that 2.6.30 doesn't need this.

Great, last I heard the changelog was still a problem.  Thanks,

-VAL
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