linux-kernel - Re: [PATCH v2] perf/core: fix a possible deadlock scenario

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Date:   Fri, 20 Jul 2018 13:52:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2] perf/core: fix a possible deadlock scenario

On Thu, Jul 19, 2018 at 12:12:53PM -0700, Cong Wang wrote:
> hrtimer_cancel() busy-waits for the hrtimer callback to stop,
> pretty much like del_timer_sync(). This creates a possible deadlock
> scenario where we hold a spinlock before calling hrtimer_cancel()
> while in trying to acquire the same spinlock in the callback.

Has this actually been observed?

> cpu_clock_event_init():
>   perf_swevent_init_hrtimer():
>     hwc->hrtimer.function = perf_swevent_hrtimer;
> 
> perf_swevent_hrtimer():
>   __perf_event_overflow():
>     __perf_event_account_interrupt():
>       perf_adjust_period():
>         pmu->stop():
>         cpu_clock_event_stop():
>           perf_swevent_cancel():
>             hrtimer_cancel()

Please explain how a hrtimer event ever gets to perf_adjust_period().
Last I checked perf_swevent_init_hrtimer() results in attr.freq=0.

> Getting stuck in an hrtimer is a disaster:

You'll get NMI watchdog splats. Getting stuck in NMI context is far more
'interesting :-)

> +#define PERF_EF_NO_WAIT	0x08		/* do not wait when stopping, for
> +					 * example, waiting for a timer
> +					 */

That's a broken comment style.