Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with load-acquire and store-release

[Peter Zijlstra]

On Mon, May 31, 2021 at 11:10:49PM +0800, Leo Yan wrote:
> Hi Peter, Adrian,
> 
> On Wed, May 19, 2021 at 10:03:19PM +0800, Leo Yan wrote:
> > 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?

Not without actual numbers; that's some terrible ifdef soup.

> 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 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.

I'm not sure wth that code is even trying to do, that's some seriously
dodgy code.