linux-kernel - [PATCH v2 3/4] sbitmap: replace CAS with atomic and

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Message-Id: <6c9b6664bcd12faa9d1a9969a32774a75d1d316f.1606058975.git.asml.silence@gmail.com>
Date:   Sun, 22 Nov 2020 15:35:47 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH v2 3/4] sbitmap: replace CAS with atomic and

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@...il.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