[tip: perf/core] perf/ring_buffer: Use local_try_cmpxchg in __perf_output_begin

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     1af61adb3a23192023fec1733bd4c8500f53e546
Gitweb:        https://git.kernel.org/tip/1af61adb3a23192023fec1733bd4c8500f53e546
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Sat, 08 Jul 2023 11:00:37 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 10 Jul 2023 09:52:36 +02:00

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: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20230708090048.63046-2-ubizjak@xxxxxxxxx
---
 kernel/events/ring_buffer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index a0433f3..fb1e180 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -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;