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sbitmap: Use atomic_long_try_cmpxchg in __sbitmap_queue_get_batch

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Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in __sbitmap_queue_get_batch.
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>
Link: https://lore.kernel.org/r/20220908151200.9993-1-ubizjak@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Uros Bizjak 2022-09-08 17:12:00 +02:00 committed by Jens Axboe
parent 1de7c3cf48
commit c35227d4e8
1 changed files with 5 additions and 5 deletions
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10
lib/sbitmap.c
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@ -533,16 +533,16 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
nr = find_first_zero_bit(&map->word, map_depth);
if (nr + nr_tags <= map_depth) {
atomic_long_t *ptr = (atomic_long_t *) &map->word;
unsigned long val, ret;
unsigned long val;
get_mask = ((1UL << nr_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,