linux-kernel - Re: BUG: using smp_processor_id() in preemptible [00000000] code: asm/8267

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Date:	Wed, 27 Mar 2013 15:02:10 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Namhyung Kim <namhyung.kim@....com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code: asm/8267

Hi Borislav,

On Tue, 26 Mar 2013 19:34:52 +0100, Borislav Petkov wrote:
> On Sun, Mar 24, 2013 at 04:59:24PM +0100, Borislav Petkov wrote:
>> On Sun, Mar 24, 2013 at 12:55:56PM +0100, Borislav Petkov wrote:
>> > Hi,
>> > 
>> > so I was tracing a small .c program like so:
>> > 
>> > $ ./perf record ~/test/asm
>> > 
>> > and here's what got spewed in dmesg. Kernel is plain -rc4. Any pending
>> > fixes in tip I should try?
>> > 
>> > [ 3700.194208] BUG: using smp_processor_id() in preemptible [00000000] code: asm/8333
>> > [ 3700.194226] caller is perf_event_task_ctx+0x55/0x140
>> 
>> Ok, here's the call stack I was able to reconstruct:
>> 
>> perf_event_exit_task()
>> |->perf_event_exit_task_context()
>>    |-> perf_event_task()
>>        |-> perf_event_task_event()
>> 	   |-> perf_event_task_ctx()
>> 	       |-> perf_event_task_match()
>> 		   |-> event_filter_match()
>> 		       |-> smp_processor_id() -> debug_smp_processor_id()
>
> Ok, jolsa just rootcaused it: It is caused by
> d610d98b5de6860feb21539726e9af7c9094151c calling perf_event_task_ctx()
> outside of the preempt-safe protection.

Oops, my bad.  Forgot to enable CONFIG_DEBUG_PREEMPT when testing. :(

>
> There's a straightforward fix below, what to people think?

Looks okay to me.  Thanks for fixing this!
Namhyung

>
> --
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7b4a55d41efc..f3bb3384a106 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4455,8 +4455,11 @@ static void perf_event_task_event(struct perf_task_event *task_event)
>  next:
>  		put_cpu_ptr(pmu->pmu_cpu_context);
>  	}
> +
> +	preempt_disable();
>  	if (task_event->task_ctx)
>  		perf_event_task_ctx(task_event->task_ctx, task_event);
> +	preempt_enable();
>  
>  	rcu_read_unlock();
>  }
--
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