Re: [PATCH] perf/ring_buffer: ensure atomicity and order of updates

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 11, 2018 at 11:59:32AM +0100, Mark Rutland wrote:
> READ_ONCE() and WRITE_ONCE() "helpfully" make a silent fallback to a
> memcpy in this case, so we're broken today, regardless of this change.
> 
> I suspect that in practice we get single-copy-atomicity for the 32-bit
> halves, and sessions likely produce less than 4GiB of ringbuffer data,
> so failures would be rare.

This should not be a problem because of the 32bit adress space limit,
which would necessarily limit us to the low word.

Also note that in perf_output_put_handle(), where we write ->data_head,
the store is from an 'unsigned long'. So on 32bit that will result in a
zero high word. Similarly, in __perf_output_begin() we read ->data_tail
into an unsigned long, which will discard the high word.

So userspace should always read (head) a zero high word, irrespective of
a split store (2x32bit), and the kernel will disregard the high word on
reading (tail), irrespective of what userspace put there.

This is all a bit subtle, and could probably use a comment, but it ought
to work..