ring_buffer: Use try_cmpxchg instead of cmpxchg

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
ring_buffer.c. 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/20230714154418.8884-1-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 52dea5d..78502d4 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -692,10 +692,7 @@ static void rb_time_set(rb_time_t *t, u64 val)
 static inline bool
 rb_time_read_cmpxchg(local_t *l, unsigned long expect, unsigned long set)
 {
-       unsigned long ret;
-
-       ret = local_cmpxchg(l, expect, set);
-       return ret == expect;
+       return local_try_cmpxchg(l, &expect, set);
 }
 
 static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
@@ -752,9 +749,7 @@ static void rb_time_set(rb_time_t *t, u64 val)
 
 static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
 {
-       u64 val;
-       val = local64_cmpxchg(&t->time, expect, set);
-       return val == expect;
+       return local64_try_cmpxchg(&t->time, &expect, set);
 }
 #endif
 
@@ -1494,14 +1489,11 @@ static bool rb_head_page_replace(struct buffer_page *old,
 {
        unsigned long *ptr = (unsigned long *)&old->list.prev->next;
        unsigned long val;
-       unsigned long ret;
 
        val = *ptr & ~RB_FLAG_MASK;
        val |= RB_PAGE_HEAD;
 
-       ret = cmpxchg(ptr, val, (unsigned long)&new->list);
-
-       return ret == val;
+       return try_cmpxchg(ptr, &val, (unsigned long)&new->list);
 }
 
 /*
@@ -3003,7 +2995,6 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
 {
        unsigned long new_index, old_index;
        struct buffer_page *bpage;
-       unsigned long index;
        unsigned long addr;
        u64 write_stamp;
        u64 delta;
@@ -3060,8 +3051,9 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
                 */
                old_index += write_mask;
                new_index += write_mask;
-               index = local_cmpxchg(&bpage->write, old_index, new_index);
-               if (index == old_index) {
+
+               /* caution: old_index gets updated on cmpxchg failure */
+               if (local_try_cmpxchg(&bpage->write, &old_index, new_index)) {
                        /* update counters */
                        local_sub(event_length, &cpu_buffer->entries_bytes);
                        return true;