hw_breakpoint: Fix Oops at destroying hw_breakpoint event on powerpc

[Michael Ellerman]

On Thu, 2016-03-03 at 11:20 +0100, Peter Zijlstra wrote:
> On Thu, Mar 03, 2016 at 08:23:38PM +1100, Michael Ellerman wrote:
> > On Wed, 2016-03-02 at 12:59 +0100, Peter Zijlstra wrote:
>
> Indeed, but if there's a preemption point in between setting and using
> that state, the ctx->task pointer might not actually still point to the
> same task. With inherited events the event might get swapped to the next
> task if it has the exact same (inherited) event configuration instead of
> reprogramming the hardware.

Yep that sounds like it would be bad for this code.

> > I can't think of a reason why we can't also store it per-cpu, but I could be
> > wrong, I don't know the code well and I haven't thought about it for very long.
>
> Right, so I'm not really up to snuff on the whole hw_breakpoint stuff
> either, that was bolted onto perf by mingo, fweisbec, kprasad and others
> while I was doing PMU bits, and I've never really dug into it.
>
> I understand kprasad is no longer with IBM, his email bounced. That's a
> shame because he knew about this stuff.. :/

I don't know that for sure but I suspect you're right, which is definitely a
shame.

> > Do you mind if I merge the following fix for now as a band-aid, and we'll try
> > and fix it up properly in the next few weeks (but maybe not in time for 4.5
> > final).
>
> OK, that works for me.
>
> Acked-by: Peter Zijlstra (Intel) <peterz at infradead.org>

Thanks. I've merged it into my fixes branch.

cheers