Re: [PATCH 2/2] perf: Remove WARN_ON_ONCE check in __perf_event_enable for valid scenario

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Jul 09, 2013 at 05:44:11PM +0200, Jiri Olsa wrote:
> The '!ctx->is_active' check has a valid scenario, so
> there's no need for the WARN macro.
> 
> The reason is that there's a time window between
> 'ctx->is_active' check in perf_event_enable function
> and __perf_event_enable function having:
>   - IRQs on
>   - ctx->lock unlocked
> 
> where the task could be killed and 'ctx' deactivated by
> perf_event_exit_task, ending up with WARN below.
> 
> Removing the WARN_ON_ONCE check.
> 
> [  324.983534] ------------[ cut here ]------------
> [  324.984420] WARNING: at kernel/events/core.c:1953 __perf_event_enable+0x187/0x190()
> [  324.984420] Modules linked in:
> [  324.984420] CPU: 19 PID: 2715 Comm: nmi_bug_snb Not tainted 3.10.0+ #246
> [  324.984420] Hardware name: Supermicro X8DTN/X8DTN, BIOS 4.6.3 01/08/2010
> [  324.984420]  0000000000000009 ffff88043fce3ec8 ffffffff8160ea0b ffff88043fce3f00
> [  324.984420]  ffffffff81080ff0 ffff8802314fdc00 ffff880231a8f800 ffff88043fcf7860
> [  324.984420]  0000000000000286 ffff880231a8f800 ffff88043fce3f10 ffffffff8108103a
> [  324.984420] Call Trace:
> [  324.984420]  <IRQ>  [<ffffffff8160ea0b>] dump_stack+0x19/0x1b
> [  324.984420]  [<ffffffff81080ff0>] warn_slowpath_common+0x70/0xa0
> [  324.984420]  [<ffffffff8108103a>] warn_slowpath_null+0x1a/0x20
> [  324.984420]  [<ffffffff81134437>] __perf_event_enable+0x187/0x190
> [  324.984420]  [<ffffffff81130030>] remote_function+0x40/0x50
> [  324.984420]  [<ffffffff810e51de>] generic_smp_call_function_single_interrupt+0xbe/0x130
> [  324.984420]  [<ffffffff81066a47>] smp_call_function_single_interrupt+0x27/0x40
> [  324.984420]  [<ffffffff8161fd2f>] call_function_single_interrupt+0x6f/0x80
> [  324.984420]  <EOI>  [<ffffffff816161a1>] ? _raw_spin_unlock_irqrestore+0x41/0x70
> [  324.984420]  [<ffffffff8113799d>] perf_event_exit_task+0x14d/0x210
> [  324.984420]  [<ffffffff810acd04>] ? switch_task_namespaces+0x24/0x60
> [  324.984420]  [<ffffffff81086946>] do_exit+0x2b6/0xa40
> [  324.984420]  [<ffffffff8161615c>] ? _raw_spin_unlock_irq+0x2c/0x30
> [  324.984420]  [<ffffffff81087279>] do_group_exit+0x49/0xc0
> [  324.984420]  [<ffffffff81096854>] get_signal_to_deliver+0x254/0x620
> [  324.984420]  [<ffffffff81043057>] do_signal+0x57/0x5a0
> [  324.984420]  [<ffffffff8161a164>] ? __do_page_fault+0x2a4/0x4e0
> [  324.984420]  [<ffffffff8161665c>] ? retint_restore_args+0xe/0xe
> [  324.984420]  [<ffffffff816166cd>] ? retint_signal+0x11/0x84
> [  324.984420]  [<ffffffff81043605>] do_notify_resume+0x65/0x80
> [  324.984420]  [<ffffffff81616702>] retint_signal+0x46/0x84
> [  324.984420] ---[ end trace 442ec2f04db3771a ]---
> 
> Reported-by: Vince Weaver <vincent.weaver@maine.edu>
> Signed-off-by: Jiri Olsa <jolsa@redhat.com>

Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Vince Weaver <vincent.weaver@maine.edu>
> ---
>  kernel/events/core.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 446ec3d..eba8fb5 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1963,7 +1963,16 @@ static int __perf_event_enable(void *info)
>  	struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
>  	int err;
>  
> -	if (WARN_ON_ONCE(!ctx->is_active))
> +	/*
> +	 * There's a time window between 'ctx->is_active' check
> +	 * in perf_event_enable function and this place having:
> +	 *   - IRQs on
> +	 *   - ctx->lock unlocked
> +	 *
> +	 * where the task could be killed and 'ctx' deactivated
> +	 * by perf_event_exit_task.
> +	 */
> +	if (!ctx->is_active)
>  		return -EINVAL;
>  
>  	raw_spin_lock(&ctx->lock);
> -- 
> 1.7.11.7
>