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

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Message-ID: <20090414164839.GI6753@linux.vnet.ibm.com>
Date:	Tue, 14 Apr 2009 09:48:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Jan Blunck <jblunck@...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 Tue, Apr 14, 2009 at 08:52:39AM +0200, Nick Piggin wrote:
> 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.
> > 
> > Signed-off-by: Jan Blunck <jblunck@...e.de>
> 
> Paul's worry about callers aside, I think it is probably a good idea
> to reduce ifdefs and share more code.

I am also OK with this patch.

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

> So for this patch,
> 
> Acked-by: Nick Piggin <npiggin@...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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