Messages in this thread | | | Date | Mon, 7 Dec 2015 17:16:19 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/7] perf: Free aux pages in unmap path |
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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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