Messages in this thread | | | Date | Fri, 11 May 2018 18:22:29 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] perf/ring_buffer: ensure atomicity and order of updates |
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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..
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