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

From: Steven Rostedt
Date: Wed Oct 04 2023 - 21:52:46 EST


From: Uros Bizjak <ubizjak@xxxxxxxxx>

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@xxxxxxxxx

Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
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