Re: [tip:perf/urgent] perf/core: Fix perf_event_disable_inatomic() race

[Peter Zijlstra]

On Wed, Apr 10, 2019 at 05:13:54AM -0700, tip-bot for Peter Zijlstra wrote:
> Commit-ID:  86071b11317550d994b55ce5e31aa06bcad783b5
> Gitweb:     https://git.kernel.org/tip/86071b11317550d994b55ce5e31aa06bcad783b5
> Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> AuthorDate: Thu, 4 Apr 2019 15:03:00 +0200
> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> CommitDate: Wed, 10 Apr 2019 13:47:09 +0200
> 
> perf/core: Fix perf_event_disable_inatomic() race
> 
> Thomas-Mich Richter reported he triggered a WARN()ing from event_function_local()
> on his s390. The problem boils down to:
> 
> 	CPU-A				CPU-B
> 
> 	perf_event_overflow()
> 	  perf_event_disable_inatomic()
> 	    @pending_disable = 1
> 	    irq_work_queue();
> 
> 	sched-out
> 	  event_sched_out()
> 	    @pending_disable = 0
> 
> 					sched-in
> 					perf_event_overflow()
> 					  perf_event_disable_inatomic()
> 					    @pending_disable = 1;
> 					    irq_work_queue(); // FAILS
> 
> 	irq_work_run()
> 	  perf_pending_event()
> 	    if (@pending_disable)
> 	      perf_event_disable_local(); // WHOOPS
> 
> The problem exists in generic, but s390 is particularly sensitive
> because it doesn't implement arch_irq_work_raise(), nor does it call
> irq_work_run() from it's PMU interrupt handler (nor would that be
> sufficient in this case, because s390 also generates
> perf_event_overflow() from pmu::stop). Add to that the fact that s390
> is a virtual architecture and (virtual) CPU-A can stall long enough
> for the above race to happen, even if it would self-IPI.
> 
> Adding a irq_work_sync() to event_sched_in() would work for all hardare
> PMUs that properly use irq_work_run() but fails for software PMUs.
> 
> Instead encode the CPU number in @pending_disable, such that we can
> tell which CPU requested the disable. This then allows us to detect
> the above scenario and even redirect the IPI to make up for the failed
> queue.

Ingo, could you please fold in the below delta? It turns out I
overlooked two insteances :-(

--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -392,7 +392,7 @@ void *perf_aux_output_begin(struct perf_
 		 * store that will be enabled on successful return
 		 */
 		if (!handle->size) { /* A, matches D */
-			event->pending_disable = 1;
+			event->pending_disable = smp_processor_id();
 			perf_output_wakeup(handle);
 			local_set(&rb->aux_nest, 0);
 			goto err_put;
@@ -480,7 +480,7 @@ void perf_aux_output_end(struct perf_out
 
 	if (wakeup) {
 		if (handle->aux_flags & PERF_AUX_FLAG_TRUNCATED)
-			handle->event->pending_disable = 1;
+			handle->event->pending_disable = smp_processor_id();
 		perf_output_wakeup(handle);
 	}