[PATCH 6.4 027/800] blk-mq: fix potential io hang by wrong wake_batch

Greg Kroah-Hartman
2023-07-16T19:55:23Z

From: Yu Kuai <yukuai3@huawei.com>

[ Upstream commit 4f1731df60f9033669f024d06ae26a6301260b55 ]

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@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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 d23a8554ec4ae..7851e149d365f 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -399,7 +399,7 @@ 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 dfd81cab57888..cc57e2dd9a0bb 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 e876584d35163..890fef9796bf9 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -417,8 +417,7 @@ 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 06caacd77ed66..710d122472641 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -746,8 +746,7 @@ 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