linux-kernel - Re: WARNING in event_function

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Date:   Wed, 13 Feb 2019 10:57:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+352bd10e338d9a90e5e0@...kaller.appspotmail.com>,
        Abderrahmane Benbachir <abderrahmane.benbachir@...ymtl.ca>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING in event_function_local

On Wed, Feb 13, 2019 at 10:51:58AM +0100, Peter Zijlstra wrote:
> On Tue, Feb 12, 2019 at 07:40:12PM -0800, Kees Cook wrote:
> 
> > > > Is this maybe just an unlucky condition with the event loop running in
> > > > an IRQ? Should the WARN be expected, or is running under an IRQ
> > > > unexpected?
> > 
> > Is perf expected to fire during an IRQ? The task == current test seems
> > suspicious if so...
> 
> So the only possible callchain here is:
> 
>   <PMI>
>     ...
>       perf_event_disable_inatomic()
>         irq_work_queue()
> 
>   <irq_work-IPI>
>     perf_pending_event()
>       perf_event_disable_local()
>         event_function_local()
> 
> 
> The assertion states that:
> 
>   if the event is a task event; and the context is active, it _must_ be
>   the same task.
> 
> Because: if the PMI happens during ctxsw (which has IRQs disabled), the
> IPI will not happen until after the ctxsw, at which point we'll also
> have switched out the perf context of that task -- IOW the context
> should be inactive.
> 
> 
> Anyway, it looks like a virt issue; I'll start caring once you can
> reproduce on real hardware.

Hurm.. I might have spoken too soon. I still don't give a crap about
virt, but I think I might see an actual problem.

The moment we re-enable IRQs after ctxsw, the task can already be
running on another CPU, and _that_ would trigger failure here.

Let me think a little about that.