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SubjectRe: [PATCH 1/2] perf/aux: Make aux_{head,wakeup} ring_buffer members long
On Mon, Jul 31, 2017 at 01:28:01PM +0300, Alexander Shishkin wrote:
> Will Deacon <will.deacon@arm.com> writes:
>
> > The aux_head and aux_wakeup members of struct ring_buffer are defined
> > using the local_t type, despite the fact that they are only accessed via
> > the perf_aux_output_* functions, which cannot race with each other for a
> > given ring buffer.
> >
> > This patch changes the type of the members to long, so we can avoid
> > using the local_* API where it isn't needed.
>
> Thanks for digging this up! Some minor nits below.
>
> > @@ -434,12 +434,12 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
> > handle->aux_flags |= PERF_AUX_FLAG_OVERWRITE;
> >
> > aux_head = handle->head;
> > - local_set(&rb->aux_head, aux_head);
> > + rb->aux_head = aux_head;
> > } else {
> > handle->aux_flags &= ~PERF_AUX_FLAG_OVERWRITE;
> >
> > - aux_head = local_read(&rb->aux_head);
> > - local_add(size, &rb->aux_head);
> > + aux_head = rb->aux_head;
> > + rb->aux_head += size;
> > }
> >
> > if (size || handle->aux_flags) {
> > @@ -451,11 +451,11 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
> > handle->aux_flags);
> > }
> >
> > - aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> > + aux_head = rb->user_page->aux_head = rb->aux_head;
> >
> > - if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> > + if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
>
> Can't we just do away with aux_head here:
>
> rb->user_page->aux_head = rb->aux_head;
> if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { ...
>
> ?
>
> > @@ -485,14 +485,13 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
> > if (size > handle->size)
> > return -ENOSPC;
> >
> > - local_add(size, &rb->aux_head);
> > + rb->aux_head += size;
> >
> > - aux_head = rb->user_page->aux_head = local_read(&rb->aux_head);
> > - if (aux_head - local_read(&rb->aux_wakeup) >= rb->aux_watermark) {
> > + aux_head = rb->user_page->aux_head = rb->aux_head;
> > + if (aux_head - rb->aux_wakeup >= rb->aux_watermark) {
> > perf_output_wakeup(handle);
> > - local_add(rb->aux_watermark, &rb->aux_wakeup);
> > - handle->wakeup = local_read(&rb->aux_wakeup) +
> > - rb->aux_watermark;
> > + rb->aux_wakeup += rb->aux_watermark;
> > + handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
> > }
> >
> > handle->head = aux_head;
>
> And here I think we don't need aux_head at all.

Agreed on both counts. Will fix for v2.

Will

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