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.
