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

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Date:	Mon, 15 Jun 2009 11:56:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Nick Piggin <npiggin@...e.de>, Jan Blunck <jblunck@...e.de>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] atomic: Fix _atomic_dec_and_lock() deadlock on UP

On Mon, Jun 15, 2009 at 02:11:13PM -0400, Valerie Aurora wrote:
> From: Jan Blunck <jblunck@...e.de>
> 
> _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.

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Jan Blunck <jblunck@...e.de>
> Signed-off-by: Valerie Aurora (Henson) <vaurora@...hat.com>
> ---
>  lib/dec_and_lock.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
> index a65c314..e73822a 100644
> --- a/lib/dec_and_lock.c
> +++ b/lib/dec_and_lock.c
> @@ -19,11 +19,10 @@
>   */
>  int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
>  {
> -#ifdef CONFIG_SMP
>  	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
>  	if (atomic_add_unless(atomic, -1, 1))
>  		return 0;
> -#endif
> +
>  	/* Otherwise do it the slow way */
>  	spin_lock(lock);
>  	if (atomic_dec_and_test(atomic))
> -- 
> 1.6.0.6
> 
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