linux-kernel - Re: x86_pmu_start WARN

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Date:	Fri, 21 Feb 2014 15:18:38 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: x86_pmu_start WARN_ON.

On Fri, 21 Feb 2014, Peter Zijlstra wrote:

> group_sched_in() that fails (for whatever reason), and without x86_pmu
> TXN support (because the leader is !x86_pmu), will corrupt the n_added
> state.
> 
> If this all is correct; the below ought to cure things.

I've applied the patch and have been unable to trigger the warning with 
either my testcase or a few hours of fuzzing.

My only comment on the patch is it could always use some comments.

The perf_event code is really hard to follow as is, without adding
more uncommented special cases.

Vince


> 
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/x86/kernel/cpu/perf_event.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 895604f2e916..79f9f848bee4 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1192,6 +1192,9 @@ static void x86_pmu_del(struct perf_event *event, int flags)
>  	for (i = 0; i < cpuc->n_events; i++) {
>  		if (event == cpuc->event_list[i]) {
>  
> +			if (i >= cpuc->n_events - cpuc->n_added)
> +				--cpuc->n_added;
> +
>  			if (x86_pmu.put_event_constraints)
>  				x86_pmu.put_event_constraints(cpuc, event);
>  
--
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