linux-kernel - Re: [PATCH 5/9] perf/x86: implement cross-HT corruption bug workaround

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Message-ID: <CAHzUATjSd0Cio25RvwNaph0vFZYQB-pKBt=xz7C=_MhZvpFXxQ@mail.gmail.com>
Date:	Thu, 5 Jun 2014 17:01:25 +0300
From:	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...ux.intel.com,
	jolsa@...hat.com, zheng.z.yan@...el.com
Subject: Re: [PATCH 5/9] perf/x86: implement cross-HT corruption bug workaround

Are you saying it is illegal to call kmalloc() from
this context?

kmalloc is needed because we need to allocate
a new constraint struct since the static constraint
cannot be modified.

Worst case we can statically allocate a second
constraint struct in the event struct.

On Thu, Jun 5, 2014 at 4:48 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Jun 04, 2014 at 11:34:14PM +0200, Stephane Eranian wrote:
>
>> +static struct event_constraint *
>> +intel_get_excl_constraints(struct cpu_hw_events *cpuc, struct perf_event *event,
>> +                        struct event_constraint *c)
>> +{
>
>> +     if (!(c->flags & PERF_X86_EVENT_DYNAMIC)) {
>> +
>> +             /*
>> +              * in case we fail, we assume no counter
>> +              * is supported to be on the safe side
>> +              */
>> +             cx = kmalloc(sizeof(*cx), GFP_KERNEL);
>> +             if (!cx)
>> +                     return &emptyconstraint;
>> +
>
> Ok, so forgive me if I'm wrong, but the way we get here is through:
>
> x86_schedule_event()
>   ->start_scheduling()
>     spin_lock()
>   ->get_event_constraints()
>     intel_get_excl_constraints()
>       kmalloc(.gfp=GFP_KERNEL)
>
> How can that ever work?
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