[PATCH -next v2] blk-mq: fix potential io hang by wrong 'wake_batch'

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

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: 180dccb0dba4 ("blk-mq: fix tag_get wait task can't be awakened")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
Changes in v2:
 - switch 'active_queues' from atomic to unsigned int.

 block/blk-mq-debugfs.c |  2 +-
 block/blk-mq-tag.c     | 15 ++++++++++-----
 block/blk-mq.h         |  3 +--
 include/linux/blk-mq.h |  3 +--
 4 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 68165a50951b..c3b5930106b2 100644
--- a/block/blk-mq-debugfs.c
+++ b/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);
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index dfd81cab5788..cc57e2dd9a0b 100644
--- a/block/blk-mq-tag.c
+++ b/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;
 
 	/*
 	 * calling test_bit() prior to test_and_set_bit() is intentional,
@@ -55,9 +56,11 @@ void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 			return;
 	}
 
-	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);
 }
 
 /*
@@ -90,9 +93,11 @@ 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);
 }
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 8c642e9f32f1..1743857e0b01 100644
--- a/block/blk-mq.h
+++ b/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;
 
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 59b52ec155b1..f401067ac03a 100644
--- a/include/linux/blk-mq.h
+++ b/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;
-- 
2.39.2