On Wed, May 15, 2019 at 01:04:16PM +0300, Alexander Shishkin wrote: > Peter Zijlstra <pet...@infradead.org> writes: > > > On Wed, May 15, 2019 at 09:51:07AM +0300, Alexander Shishkin wrote: > >> Yabin Cui <yab...@google.com> writes: > >> > >> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > >> > index 674b35383491..0b9aefe13b04 100644 > >> > --- a/kernel/events/ring_buffer.c > >> > +++ b/kernel/events/ring_buffer.c > >> > @@ -54,8 +54,10 @@ static void perf_output_put_handle(struct > >> > perf_output_handle *handle) > >> > * IRQ/NMI can happen here, which means we can miss a head > >> > update. > >> > */ > >> > > >> > - if (!local_dec_and_test(&rb->nest)) > >> > + if (local_read(&rb->nest) > 1) { > >> > + local_dec(&rb->nest); > >> > >> What stops rb->nest changing between local_read() and local_dec()? > > > > Nothing, however it must remain the same :-) > > > > That is the cryptic way of saying that since these buffers are strictly > > per-cpu, the only change can come from interrupts, and they must have a > > net 0 change. Or rather, an equal amount of decrements to increments. > > > > So if it changes, it must also change back to where it was. > > Ah that's true. So the whole ->nest thing can be done with > READ_ONCE()/WRITE_ONCE() instead? > Because the use of local_dec_and_test() creates an impression that we > rely on atomicity of it, which in actuality we don't.
Yes, I think we can get away with that. And that might be a worth-while optimization for !x86.