[PATCH v2] ring_buffer: Use try_cmpxchg instead of cmpxchg
DORMANTno replies
LANDED
From: Uros Bizjak <hidden>
Date: 2023-07-14 15:44:58
Also in:
lkml
Subsystem:
the rest, tracing · Maintainers:
Linus Torvalds, Steven Rostedt, Masami Hiramatsu
Landed in mainline as 00a8478f8f5c on 2023-07-30.
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. Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Signed-off-by: Uros Bizjak <redacted> --- kernel/trace/ring_buffer.c | 20 ++++++-------------- 1 file changed, 6 insertions(+), 14 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 834b361a4a66..84e3235ae8c7 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c@@ -688,10 +688,7 @@ 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)
@@ -748,9 +745,7 @@ 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
@@ -1489,14 +1484,11 @@ 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); } /*
@@ -3000,7 +2992,6 @@ 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;
@@ -3057,8 +3048,9 @@ 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;
--
2.41.0