Commit 1af61adb authored by Uros Bizjak's avatar Uros Bizjak Committed by Peter Zijlstra
Browse files

perf/ring_buffer: Use local_try_cmpxchg in __perf_output_begin 



Use local_try_cmpxchg instead of local_cmpxchg (*ptr, old, new) == old
in __perf_output_begin.  x86 CMPXCHG instruction returns success in ZF
flag, so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230708090048.63046-2-ubizjak@gmail.com
parent d6b45484

kernel/events/ring_buffer.c

+3 −2
Original line number Diff line number Diff line
@@ -191,9 +191,10 @@ __perf_output_begin(struct perf_output_handle *handle, 


	perf_output_get_handle(handle);



	offset = local_read(&rb->head);

	do {

		head = offset;

		tail = READ_ONCE(rb->user_page->data_tail);

		offset = head = local_read(&rb->head);

		if (!rb->overwrite) {

			if (unlikely(!ring_buffer_has_space(head, tail,

							    perf_data_size(rb),
@@ -217,7 +218,7 @@ __perf_output_begin(struct perf_output_handle *handle, 
			head += size;

		else

			head -= size;

	} while (local_cmpxchg(&rb->head, offset, head) != offset);

	} while (!local_try_cmpxchg(&rb->head, &offset, head));



	if (backward) {

		offset = head;