Message ID | 20230305205628.27385-11-ubizjak@gmail.com |
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State | New |
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Series |
locking: Introduce local{,64}_try_cmpxchg
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Commit Message
Uros Bizjak
March 5, 2023, 8:56 p.m. UTC
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.
Also, local_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.
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
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 273a0fe7910a..e07c10f4d141 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;