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

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Peter Zijlstra <peterz@infradead.org> writes:

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

Regards,
--
Alex