linux-kernel - [PATCH] ring_buffer: Use try_cmpxchg instead of cmpxchg in rb_insert

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Date:   Thu, 14 Sep 2023 18:34:02 +0200
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] ring_buffer: Use try_cmpxchg instead of cmpxchg in rb_insert_pages

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

No functional change intended.

Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 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 a1651edc48d5..3e2a6478425c 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2048,7 +2048,7 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
 	retries = 10;
 	success = false;
 	while (retries--) {
-		struct list_head *head_page, *prev_page, *r;
+		struct list_head *head_page, *prev_page;
 		struct list_head *last_page, *first_page;
 		struct list_head *head_page_with_bit;
 		struct buffer_page *hpage = rb_set_head_page(cpu_buffer);
@@ -2067,9 +2067,9 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
 		last_page->next = head_page_with_bit;
 		first_page->prev = prev_page;
 
-		r = cmpxchg(&prev_page->next, head_page_with_bit, first_page);
-
-		if (r == head_page_with_bit) {
+		/* caution: head_page_with_bit gets updated on cmpxchg failure */
+		if (try_cmpxchg(&prev_page->next,
+				&head_page_with_bit, first_page)) {
 			/*
 			 * yay, we replaced the page pointer to our new list,
 			 * now, we just have to update to head page's prev
-- 
2.41.0