Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.

[Peter Zijlstra]

On Wed, May 15, 2019 at 09:51:07AM +0300, Alexander Shishkin wrote:
> Yabin Cui <yabinc@xxxxxxxxxx> 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.