linux-kernel - RE: [RFC][PATCH] irq_work: Don't ignore possible cmpxchg failure

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Date:	Tue, 16 Nov 2010 18:08:46 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Aguirre, Sergio" <saaguirre@...com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Huang Ying <ying.huang@...el.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, Kyle McMartin <kyle@...artin.ca>
Subject: RE: [RFC][PATCH] irq_work: Don't ignore possible cmpxchg failure

On Tue, 2010-11-16 at 10:57 -0600, Aguirre, Sergio wrote:

> > > @@ -145,7 +145,10 @@ void irq_work_run(void)
> > >  		 * Clear the BUSY bit and return to the free state if
> > >  		 * no-one else claimed it meanwhile.
> > >  		 */
> > > -		cmpxchg(&entry->next, next_flags(NULL, IRQ_WORK_BUSY), NULL);
> > > +		xchgres = cmpxchg(&entry->next,
> > > +				  next_flags(NULL, IRQ_WORK_BUSY),
> > > +				  NULL);
> > > +		BUG_ON(unlikely(xchgres != next_flags(NULL, IRQ_WORK_BUSY)));
> > 
> > simply adding (void) in front would be much easier.
> 
> But isn't that still leaving the remote possibility of a hidden cmpxchg
> Failure open?

No, we don't care if it fails, read the comment. All we want to know is
that if it still matched, we flipped the bit.
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