Peter Zijlstra <[email protected]> writes: > On Wed, May 15, 2019 at 09:51:07AM +0300, Alexander Shishkin wrote: >> Yabin Cui <[email protected]> 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. Regards, -- Alex
