On Sat 10-06-23 10:30:43, Yu Kuai wrote:
> 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>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> 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
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
