Re: WARNING in event_function_local

[Peter Zijlstra <peterz@infradead.org>]

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.