[PATCH v2 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg

From: Uros Bizjak
Date: Thu Mar 02 2023 - 11:42:06 EST


Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old.
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.

Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
v2: Convert only loops with cmpxchg.
---
kernel/trace/ring_buffer.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 4188af7d4cfe..9a6ba5824cf2 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4061,10 +4061,10 @@ void ring_buffer_record_off(struct trace_buffer *buffer)
unsigned int rd;
unsigned int new_rd;

+ rd = atomic_read(&buffer->record_disabled);
do {
- rd = atomic_read(&buffer->record_disabled);
new_rd = rd | RB_BUFFER_OFF;
- } while (atomic_cmpxchg(&buffer->record_disabled, rd, new_rd) != rd);
+ } while (!atomic_try_cmpxchg(&buffer->record_disabled, &rd, new_rd));
}
EXPORT_SYMBOL_GPL(ring_buffer_record_off);

@@ -4084,10 +4084,10 @@ void ring_buffer_record_on(struct trace_buffer *buffer)
unsigned int rd;
unsigned int new_rd;

+ rd = atomic_read(&buffer->record_disabled);
do {
- rd = atomic_read(&buffer->record_disabled);
new_rd = rd & ~RB_BUFFER_OFF;
- } while (atomic_cmpxchg(&buffer->record_disabled, rd, new_rd) != rd);
+ } while (!atomic_try_cmpxchg(&buffer->record_disabled, &rd, new_rd));
}
EXPORT_SYMBOL_GPL(ring_buffer_record_on);

--
2.39.2