On Fri, Dec 04, 2015 at 11:17:23PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 04, 2015 at 06:02:06PM +0100, Peter Zijlstra wrote:
> > The only solution I've come up with is:
> >
> > struct rb_aux *aux = rb->aux;
> >
> > if (aux && vma->vm_pgoff == aux->pgoff) {
> > ctx = perf_event_ctx_lock(event);
>
> Can't do this at all, see the comment in put_event(). perf_read_group()
> accesses user memory (and hence causes faults, which in turn take
> mmap_sem) while holding ctx::mutex.
>
> So neither this, not what you proposed can work.
>
> Will need moar thinking.
So we could try and see if we can get this working:
static int __perf_event_stop(void *info)
{
struct perf_event *event = info;
/* IRQs disabled, cannot get scheduled away */
if (event->oncpu == smp_processor_id()) {
event->pmu->stop(event);
return 0;
}
return -EAGAIN;
}
perf_event_stop(struct perf_event *event)
{
for (;;) {
if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE)
break;
smp_rmb(); /* if we see ACTIVE, ->oncpu must be set */
if (!cpu_function_call(READ_ONCE(event->oncpu),
__perf_event_stop, event))
break;
}
}
That probably wants some WRITE_ONCE() and maybe some memory barriers in
event_sched_in() as well, like:
WRITE_ONCE(event->oncpu, smp_processor_id());
smp_wmb();
WRITE_ONCE(event->state, PERF_EVENT_STATE_ACTIVE);
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