[PATCH RESEND] sbitmap: Use atomic_{,long}_try_cmpxchg in sbitmap.c

Uros Bizjak
2022-09-07T18:22:38Z

Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in __sbitmap_queue_get_batch
and atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in __sbq_wake_up. x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg).

Also, atomic_long_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications, e.g.
an extra memory read can be avoided in the loop.

No functional change intended.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
lib/sbitmap.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 29eb0484215a..32392e65b98f 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -534,16 +534,16 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
if (nr + nr_tags <= map_depth) {
atomic_long_t *ptr = (atomic_long_t *) &map->word;
int map_tags = min_t(int, nr_tags, map_depth);
- unsigned long val, ret;
+ unsigned long val;

get_mask = ((1UL << map_tags) - 1) << nr;
+ val = READ_ONCE(map->word);
do {
- val = READ_ONCE(map->word);
if ((val & ~get_mask) != val)
goto next;
- ret = atomic_long_cmpxchg(ptr, val, get_mask | val);
- } while (ret != val);
- get_mask = (get_mask & ~ret) >> nr;
+ } while (!atomic_long_try_cmpxchg(ptr, &val,
+ get_mask | val));
+ get_mask = (get_mask & ~val) >> nr;
if (get_mask) {
*offset = nr + (index << sb->shift);
update_alloc_hint_after_get(sb, depth, hint,
@@ -612,8 +612,6 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)

wait_cnt = atomic_dec_return(&ws->wait_cnt);
if (wait_cnt <= 0) {
- int ret;
-
wake_batch = READ_ONCE(sbq->wake_batch);

/*
@@ -628,8 +626,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
* atomic_cmpxhcg() race should call this function again
* to wakeup a new batch on a different 'ws'.
*/
- ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
- if (ret == wait_cnt) {
+ if (atomic_try_cmpxchg(&ws->wait_cnt, &wait_cnt, wake_batch)) {
sbq_index_atomic_inc(&sbq->wake_index);
wake_up_nr(&ws->wait, wake_batch);
return false;
--
2.37.2