blk-mq: fix potential io hang by wrong 'wake_batch'

In __blk_mq_tag_busy/idle(), updating 'active_queues' and calculating
'wake_batch' is not atomic:

t1:			t2:
_blk_mq_tag_busy	blk_mq_tag_busy
inc active_queues
// assume 1->2
			inc active_queues
			// 2 -> 3
			blk_mq_update_wake_batch
			// calculate based on 3
blk_mq_update_wake_batch
/* calculate based on 2, while active_queues is actually 3. */

Fix this problem by protecting them wih 'tags->lock', this is not a hot
path, so performance should not be concerned. And now that all writers
are inside the lock, switch 'actives_queues' from atomic to unsigned
int.

Fixes: 180dccb ("blk-mq: fix tag_get wait task can't be awakened")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230610023043.2559121-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-debugfs.c

@@ -401,7 +401,7 @@ static void blk_mq_debugfs_tags_show(struct seq_file *m,
 	seq_printf(m, "nr_tags=%u\n", tags->nr_tags);
 	seq_printf(m, "nr_reserved_tags=%u\n", tags->nr_reserved_tags);
 	seq_printf(m, "active_queues=%d\n",
-		   atomic_read(&tags->active_queues));
+		   READ_ONCE(tags->active_queues));
 
 	seq_puts(m, "\nbitmap_tags:\n");
 	sbitmap_queue_show(&tags->bitmap_tags, m);

block/blk-mq-tag.c

@@ -38,6 +38,7 @@ static void blk_mq_update_wake_batch(struct blk_mq_tags *tags,
 void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 {
 	unsigned int users;
+	struct blk_mq_tags *tags = hctx->tags;
 
 	if (blk_mq_is_shared_tags(hctx->flags)) {
 		struct request_queue *q = hctx->queue;
@@ -51,9 +52,11 @@ void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 		set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state);
 	}
 
-	users = atomic_inc_return(&hctx->tags->active_queues);
-
-	blk_mq_update_wake_batch(hctx->tags, users);
+	spin_lock_irq(&tags->lock);
+	users = tags->active_queues + 1;
+	WRITE_ONCE(tags->active_queues, users);
+	blk_mq_update_wake_batch(tags, users);
+	spin_unlock_irq(&tags->lock);
 }
 
 /*
@@ -86,9 +89,11 @@ void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
 			return;
 	}
 
-	users = atomic_dec_return(&tags->active_queues);
-
+	spin_lock_irq(&tags->lock);
+	users = tags->active_queues - 1;
+	WRITE_ONCE(tags->active_queues, users);
 	blk_mq_update_wake_batch(tags, users);
+	spin_unlock_irq(&tags->lock);
 
 	blk_mq_tag_wakeup_all(tags, false);
 }

block/blk-mq.h

@@ -412,8 +412,7 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
 			return true;
 	}
 
-	users = atomic_read(&hctx->tags->active_queues);
-
+	users = READ_ONCE(hctx->tags->active_queues);
 	if (!users)
 		return true;
 

include/linux/blk-mq.h

@@ -739,8 +739,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
 struct blk_mq_tags {
 	unsigned int nr_tags;
 	unsigned int nr_reserved_tags;
-
-	atomic_t active_queues;
+	unsigned int active_queues;
 
 	struct sbitmap_queue bitmap_tags;
 	struct sbitmap_queue breserved_tags;