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.

Link: https://lore.kernel.org/linux-trace-kernel/20230914163420.12923-1-ubizjak@gmail.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 515cafdb18d98a0f6846f078847bead4c8ecd87d..43cc47d7faafc7bd9c80d62fe4ad03f97b4164eb 100644 (file)
--- 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