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Date: Wed, 10 Aug 2016 13:04:38 +0200
From: Ingo Molnar <mingo@kernel.org>
To: David Carrillo-Cisneros <davidcc@google.com>
Cc: linux-kernel@vger.kernel.org, "x86@kernel.org" <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Andi Kleen <ak@linux.intel.com>, Kan Liang <kan.liang@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vegard Nossum <vegard.nossum@gmail.com>, Paul Turner <pjt@google.com>,
        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v2 1/4] perf/core: check return value of perf_event_read
 IPI
Message-ID: <20160810110437.GB14283@gmail.com>
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* David Carrillo-Cisneros <davidcc@google.com> wrote:

> The call to smp_call_function_single in perf_event_read() may fail and,
> when it does, its error value is the one to return.
> 
> Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
> Reviewed-by: Stephane Eranian <eranian@google.com>
> ---
>  kernel/events/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9345028..96523fd 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3469,9 +3469,9 @@ static int perf_event_read(struct perf_event *event, bool group)
>  			.group = group,
>  			.ret = 0,
>  		};
> -		smp_call_function_single(event->oncpu,
> -					 __perf_event_read, &data, 1);
> -		ret = data.ret;
> +		ret = smp_call_function_single(event->oncpu,
> +					       __perf_event_read, &data, 1);
> +		ret = ret ? : data.ret;
>  	} else if (event->state == PERF_EVENT_STATE_INACTIVE) {
>  		struct perf_event_context *ctx = event->ctx;
>  		unsigned long flags;

Please also describe the symptoms of the bug - under what circumstances may the 
smp-call fail and how does the user notice?

Thanks,

        Ingo