linux-kernel - Re: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well

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Message-ID: <20090411174905.GH6822@linux.vnet.ibm.com>
Date:	Sat, 11 Apr 2009 10:49:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jan Blunck <jblunck@...e.de>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] atomic: Only take lock when the counter drops to zero
	on UP as well

On Fri, Apr 10, 2009 at 06:13:57PM +0200, Jan Blunck wrote:
> I think it is wrong to unconditionally take the lock before calling
> atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in
> situation where it is known that the counter will not reach zero (e.g. holding
> another reference to the same object) but the lock is already taken.

The thought of calling _atomic_dec_and_lock() when you already hold the
lock really really scares me.

Could you please give an example where you need to do this?

							Thanx, Paul

> Signed-off-by: Jan Blunck <jblunck@...e.de>
> ---
>  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))
> -- 
> 1.6.0.2
> 
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