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From: Peter Zijlstra on 19 May 2010 02:40
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(a)redhat.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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