linux-kernel - Re: [PATCH] perf: fix cmpxchg warning in perf_event

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Date:	Wed, 19 May 2010 08:32:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	eranian@...glemail.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, paulus@...ba.org, perfmon2-devel@...ts.sf.net
Subject: Re: [PATCH] perf: fix cmpxchg warning in perf_event_amd.c

On Tue, 2010-05-18 at 16:43 -0400, Jason Baron wrote:
> Hi,
> 
> I'm getting the following warnings:
> 
> In file included from arch/x86/kernel/cpu/perf_event.c:1343:
> arch/x86/kernel/cpu/perf_event_amd.c: In function
> ‘amd_put_event_constraints’:
> arch/x86/kernel/cpu/perf_event_amd.c:167: warning: value computed is not
> used
> 
> Since cmpxchg returns a value, we need to use it. We can use it to test if its
> equal to "event".

Should we:

 a) add a WARN_ONCE() there and make use of the return value (it should
never happen)

or

 b) replace the lot with: nb->owners[i] = NULL; which should I _think_
work.

> Signed-off-by: Jason Baron <jbaron@...hat.com>
> ---
>  arch/x86/kernel/cpu/perf_event_amd.c |    4 +---
>  1 files changed, 1 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_amd.c b/arch/x86/kernel/cpu/perf_event_amd.c
> index 611df11..e13ce4a 100644
> --- a/arch/x86/kernel/cpu/perf_event_amd.c
> +++ b/arch/x86/kernel/cpu/perf_event_amd.c
> @@ -163,10 +163,8 @@ static void amd_put_event_constraints(struct cpu_hw_events *cpuc,
>  	 * when we come here
>  	 */
>  	for (i = 0; i < x86_pmu.num_counters; i++) {
> -		if (nb->owners[i] == event) {
> -			cmpxchg(nb->owners+i, event, NULL);
> +		if (cmpxchg(nb->owners+i, event, NULL) == event)
>  			break;
> -		}
>  	}
>  }
>  

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