On 31/05/21 6:10 pm, Leo Yan wrote:

Hi Peter, Adrian,

On Wed, May 19, 2021 at 10:03:19PM +0800, Leo Yan wrote:

quoted

Load-acquire and store-release are one-way permeable barriers, which can
be used to guarantee the memory ordering between accessing the buffer
data and the buffer's head / tail.

This patch optimizes the memory ordering with the load-acquire and
store-release barriers.

Is this patch okay for you?

Besides this patch, I have an extra question.  You could see for
accessing the AUX buffer's head and tail, it also support to use
compiler build-in functions for atomicity accessing:

  __sync_val_compare_and_swap()
  __sync_bool_compare_and_swap()

Since now we have READ_ONCE()/WRITE_ONCE(), do you think we still need
to support __sync_xxx_compare_and_swap() atomicity?

I don't remember, but it seems to me atomicity is needed only
for a 32-bit perf running with a 64-bit kernel.

I checked the code for updating head and tail for the perf ring buffer
(see ring_buffer_read_head() and ring_buffer_write_tail() in the file
tools/include/linux/ring_buffer.h), which doesn't support
__sync_xxx_compare_and_swap() anymore.  This is why I wander if should
drop __sync_xxx_compare_and_swap() atomicity for AUX ring buffer as
well.

Thanks,
Leo