Revert "sbitmap: fix batched wait_cnt accounting"

This reverts commit 16ede669.

This is causing issues with CPU stalls on my test box, revert it for
now until we understand what is going on. It looks like infinite
looping off sbitmap_queue_wake_up(), but hard to tell with a lot of
CPUs hitting this issue and the console scrolling infinitely.

Link: https://lore.kernel.org/linux-block/e742813b-ce5c-0d58-205b-1626f639b1bd@kernel.dk/Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-tag.c

@@ -196,7 +196,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 		 * other allocations on previous queue won't be starved.
 		 */
 		if (bt != bt_prev)
-			sbitmap_queue_wake_up(bt_prev, 1);
+			sbitmap_queue_wake_up(bt_prev);
 
 		ws = bt_wait_ptr(bt, data->hctx);
 	} while (1);

include/linux/sbitmap.h

@@ -575,9 +575,8 @@ void sbitmap_queue_wake_all(struct sbitmap_queue *sbq);
  * sbitmap_queue_wake_up() - Wake up some of waiters in one waitqueue
  * on a &struct sbitmap_queue.
  * @sbq: Bitmap queue to wake up.
- * @nr: Number of bits cleared.
  */
-void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr);
+void sbitmap_queue_wake_up(struct sbitmap_queue *sbq);
 
 /**
  * sbitmap_queue_show() - Dump &struct sbitmap_queue information to a &struct

lib/sbitmap.c

@@ -599,38 +599,34 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
 	return NULL;
 }
 
-static bool __sbq_wake_up(struct sbitmap_queue *sbq, int nr)
+static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 {
 	struct sbq_wait_state *ws;
-	int wake_batch, wait_cnt, cur;
+	unsigned int wake_batch;
+	int wait_cnt;
 
 	ws = sbq_wake_ptr(sbq);
-	if (!ws || !nr)
+	if (!ws)
 		return false;
 
-	wake_batch = READ_ONCE(sbq->wake_batch);
-	cur = atomic_read(&ws->wait_cnt);
-	do {
-		if (cur <= 0)
-			return true;
-		wait_cnt = cur - nr;
-	} while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt));
-
+	wait_cnt = atomic_dec_return(&ws->wait_cnt);
 	/*
 	 * For concurrent callers of this, callers should call this function
 	 * again to wakeup a new batch on a different 'ws'.
 	 */
-	if (!waitqueue_active(&ws->wait))
+	if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
 		return true;
 
 	if (wait_cnt > 0)
 		return false;
 
+	wake_batch = READ_ONCE(sbq->wake_batch);
+
 	/*
 	 * Wake up first in case that concurrent callers decrease wait_cnt
 	 * while waitqueue is empty.
 	 */
-	wake_up_nr(&ws->wait, max(wake_batch, nr));
+	wake_up_nr(&ws->wait, wake_batch);
 
 	/*
 	 * Pairs with the memory barrier in sbitmap_queue_resize() to
@@ -655,11 +651,12 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int nr)
 	return false;
 }
 
-void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
+void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
 {
-	while (__sbq_wake_up(sbq, nr))
+	while (__sbq_wake_up(sbq))
 		;
 }
+EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
 
 static inline void sbitmap_update_cpu_hint(struct sbitmap *sb, int cpu, int tag)
 {
@@ -696,7 +693,7 @@ void sbitmap_queue_clear_batch(struct sbitmap_queue *sbq, int offset,
 		atomic_long_andnot(mask, (atomic_long_t *) addr);
 
 	smp_mb__after_atomic();
-	sbitmap_queue_wake_up(sbq, nr_tags);
+	sbitmap_queue_wake_up(sbq);
 	sbitmap_update_cpu_hint(&sbq->sb, raw_smp_processor_id(),
 					tags[nr_tags - 1] - offset);
 }
@@ -724,7 +721,7 @@ void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr,
 	 * waiter. See the comment on waitqueue_active().
 	 */
 	smp_mb__after_atomic();
-	sbitmap_queue_wake_up(sbq, 1);
+	sbitmap_queue_wake_up(sbq);
 	sbitmap_update_cpu_hint(&sbq->sb, cpu, nr);
 }
 EXPORT_SYMBOL_GPL(sbitmap_queue_clear);