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Subject[PATCH v2 3/4] sbitmap: replace CAS with atomic and
Date
sbitmap_deferred_clear() does CAS loop to propagate cleared bits,
replace it with equivalent atomic bitwise and. That's slightly faster
and makes wait-free instead of lock-free as before.

The atomic can be relaxed (i.e. barrier-less) because following
sbitmap_get*() deal with synchronisation, see comments in
sbitmap_queue_clear().

It's ok to cast to atomic_long_t, that's what bitops/lock.h does.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
lib/sbitmap.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 4fd877048ba8..c18b518a16ba 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -14,7 +14,7 @@
*/
static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
{
- unsigned long mask, val;
+ unsigned long mask;

if (!READ_ONCE(map->cleared))
return false;
@@ -27,10 +27,8 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
/*
* Now clear the masked bits in our free word
*/
- do {
- val = map->word;
- } while (cmpxchg(&map->word, val, val & ~mask) != val);
-
+ atomic_long_andnot(mask, (atomic_long_t *)&map->word);
+ BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
return true;
}

--
2.24.0
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