linux-kernel - [PATCH v4 3/3] ring_buffer: Use try

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Date:   Sun,  5 Mar 2023 16:55:32 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: [PATCH v4 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg

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@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
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 5235037f83d3..d17345b522f4 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