linux-kernel - [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

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Message-Id: <722e1d561f0a49dc464d3a2b1be4c077f7502726.1558625912.git.asml.silence@gmail.com>
Date:   Thu, 23 May 2019 18:39:16 +0300
From:   "Pavel Begunkov (Silence)" <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

From: Pavel Begunkov <asml.silence@...il.com>

cmpxchg() with an immediate value could be replaced with less expensive
xchg(). The same true if new value don't _depend_ on the old one.

In the second block, atomic_cmpxchg() return value isn't checked, so
after atomic_cmpxchg() ->  atomic_xchg() conversion it could be replaced
with atomic_set(). Comparison with atomic_read() in the second chunk was
left as an optimisation (if that was the initial intention).

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 lib/sbitmap.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 155fe38756ec..7d7e0e278523 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -37,9 +37,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
 	/*
 	 * First get a stable cleared mask, setting the old mask to 0.
 	 */
-	do {
-		mask = sb->map[index].cleared;
-	} while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask);
+	mask = xchg(&sb->map[index].cleared, 0);
 
 	/*
 	 * Now clear the masked bits in our free word
@@ -527,10 +525,8 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
 		struct sbq_wait_state *ws = &sbq->ws[wake_index];
 
 		if (waitqueue_active(&ws->wait)) {
-			int o = atomic_read(&sbq->wake_index);
-
-			if (wake_index != o)
-				atomic_cmpxchg(&sbq->wake_index, o, wake_index);
+			if (wake_index != atomic_read(&sbq->wake_index))
+				atomic_set(&sbq->wake_index, wake_index);
 			return ws;
 		}
 
-- 
2.21.0