From: Cong Wang <xiyou.wangcong@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] perf/core: fix a possible deadlock scenario
Date: Mon, 23 Jul 2018 16:16:23 -0700 [thread overview]
Message-ID: <CAM_iQpWVhLZKo9j6COGkvryGrxJq6sEXzh2S-ikza2H3=pAyqw@mail.gmail.com> (raw)
In-Reply-To: <20180720115217.GQ2494@hirez.programming.kicks-ass.net>
On Fri, Jul 20, 2018 at 4:52 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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?
Without lockdep annotation, it is not easy to observe.
>
> > 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.
Good point.
I thought attr.freq is specified by user-space, but seems
perf_swevent_init_hrtimer() clears it purposely and will not change
after initialization, interesting...
>
> > Getting stuck in an hrtimer is a disaster:
>
> You'll get NMI watchdog splats. Getting stuck in NMI context is far more
> 'interesting :-)
Yes, I did see a stack trace in perf_swevent_hrtimer() which led
me here. But I have to admit among those hundreds of soft lockup's,
I only saw one showing swevent hrtimer backtrace.
Previously I thought this is because of NMI handler race, but Jiri
pointed out the race doesn't exist.
>
> > +#define PERF_EF_NO_WAIT 0x08 /* do not wait when stopping, for
> > + * example, waiting for a timer
> > + */
>
> That's a broken comment style.
It is picked by checkpatch.pl, not me, I chose a different one and got
a complain. :)
Thanks!
next prev parent reply other threads:[~2018-07-23 23:16 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-16 21:51 [PATCH] perf/core: fix a possible deadlock scenario Cong Wang
2018-07-18 8:19 ` Jiri Olsa
2018-07-18 20:21 ` Cong Wang
2018-07-19 13:28 ` Jiri Olsa
2018-07-19 19:12 ` [PATCH v2] " Cong Wang
2018-07-20 11:52 ` Peter Zijlstra
2018-07-23 23:16 ` Cong Wang [this message]
2018-07-24 1:35 ` Cong Wang
2018-07-24 1:44 ` Cong Wang
2018-07-24 9:18 ` Peter Zijlstra
2018-07-25 18:44 ` Cong Wang
2018-07-24 9:12 ` Peter Zijlstra
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