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

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Date:	Fri, 17 Apr 2009 15:14:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Blunck <jblunck@...e.de>
Cc:	npiggin@...e.de, paulmck@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] atomic: Only take lock when the counter drops to zero
 on UP as well

On Fri, 10 Apr 2009 18:13:57 +0200
Jan Blunck <jblunck@...e.de> 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.
> 

It can't deadlock, because spin_lock() doesn't do anything on
CONFIG_SMP=n.

You might get lockdep whines on CONFIG_SMP=n, but they'd be false
positives because lockdep doesn't know that we generate additional code
for SMP builds.

> ---
>  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))

The patch looks reasonable from a cleanup/consistency POV, but the
analysis and changelog need a bit of help, methinks.

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