linux-kernel - [for-next][PATCH 2/7] ring_buffer: Use try_cmpxchg instead of cmpxchg in rb_insert

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Date:   Wed, 04 Oct 2023 21:53:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uros Bizjak <ubizjak@...il.com>
Subject: [for-next][PATCH 2/7] ring_buffer: Use try_cmpxchg instead of cmpxchg in rb_insert_pages

From: Uros Bizjak <ubizjak@...il.com>

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.

Link: https://lore.kernel.org/linux-trace-kernel/20230914163420.12923-1-ubizjak@gmail.com

Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 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 515cafdb18d9..43cc47d7faaf 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2056,7 +2056,7 @@ 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);
@@ -2075,9 +2075,9 @@ 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
-- 
2.40.1