linux-kernel - [PATCH 5/6] sbitmap: wake_index doesn't need to be atomic

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Date:   Thu, 20 Jul 2023 17:45:54 +0800
From:   chengming.zhou@...ux.dev
To:     axboe@...nel.dk, osandov@...com, ming.lei@...hat.com,
        kbusch@...nel.org, krisman@...e.de
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        zhouchengming@...edance.com
Subject: [PATCH 5/6] sbitmap: wake_index doesn't need to be atomic_t

From: Chengming Zhou <zhouchengming@...edance.com>

We use wake_index to remember from which to wake up next time, which
doesn't need to be atomic_t since we only read it once before wakeups,
and write it once after wakeups.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 include/linux/sbitmap.h |  2 +-
 lib/sbitmap.c           | 12 ++++++------
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index d662cf136021..bdbe478ba4dc 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -116,7 +116,7 @@ struct sbitmap_queue {
 	/**
 	 * @wake_index: Next wait queue in @ws to wake up.
 	 */
-	atomic_t wake_index;
+	unsigned int wake_index;
 
 	/**
 	 * @ws: Wait queues.
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 50bdf3a31947..6778ab3fc6a5 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -419,7 +419,7 @@ int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,
 
 	sbq->min_shallow_depth = UINT_MAX;
 	sbq->wake_batch = sbq_calc_wake_batch(sbq, depth);
-	atomic_set(&sbq->wake_index, 0);
+	sbq->wake_index = 0;
 	atomic_set(&sbq->ws_active, 0);
 	atomic_set(&sbq->completion_cnt, 0);
 	atomic_set(&sbq->wakeup_cnt, 0);
@@ -549,7 +549,7 @@ static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
 	if (!atomic_read(&sbq->ws_active))
 		return;
 
-	wake_index = atomic_read(&sbq->wake_index);
+	wake_index = READ_ONCE(sbq->wake_index);
 	for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
 		struct sbq_wait_state *ws = &sbq->ws[wake_index];
 
@@ -570,8 +570,8 @@ static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
 			break;
 	}
 
-	if (wake_index != atomic_read(&sbq->wake_index))
-		atomic_set(&sbq->wake_index, wake_index);
+	if (wake_index != READ_ONCE(sbq->wake_index))
+		WRITE_ONCE(sbq->wake_index, wake_index);
 }
 
 void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
@@ -672,7 +672,7 @@ void sbitmap_queue_wake_all(struct sbitmap_queue *sbq)
 	 * sbitmap_queue_wake_up().
 	 */
 	smp_mb();
-	wake_index = atomic_read(&sbq->wake_index);
+	wake_index = READ_ONCE(sbq->wake_index);
 	for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
 		struct sbq_wait_state *ws = &sbq->ws[wake_index];
 
@@ -702,7 +702,7 @@ void sbitmap_queue_show(struct sbitmap_queue *sbq, struct seq_file *m)
 	seq_puts(m, "}\n");
 
 	seq_printf(m, "wake_batch=%u\n", sbq->wake_batch);
-	seq_printf(m, "wake_index=%d\n", atomic_read(&sbq->wake_index));
+	seq_printf(m, "wake_index=%d\n", READ_ONCE(sbq->wake_index));
 	seq_printf(m, "ws_active=%d\n", atomic_read(&sbq->ws_active));
 
 	seq_puts(m, "ws={\n");
-- 
2.41.0