Messages in this thread Patch in this message | | | From | Roman Pen <> | Subject | [PATCH v2 1/1] blk-mq: fix hang caused by freeze/unfreeze sequence | Date | Mon, 8 Aug 2016 13:39:08 +0200 |
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Long time ago there was a similar fix proposed by Akinobu Mita[1], but it seems that time everyone decided to fix this subtle race in percpu-refcount and Tejun Heo[2] did an attempt (as I can see that patchset was not applied).
The following is a description of a hang in blk_mq_freeze_queue_wait() - same fix but a bug from another angle.
The hang happens on attempt to freeze a queue while another task does queue unfreeze.
The root cause is an incorrect sequence of percpu_ref_reinit() and percpu_ref_kill() and as a result those two can be swapped:
CPU#0 CPU#1 ---------------- ----------------- percpu_ref_kill()
percpu_ref_kill() << atomic reference does percpu_ref_reinit() << not guarantee the order
blk_mq_freeze_queue_wait() << HANG HERE
percpu_ref_reinit()
Firstly this wrong sequence raises two kernel warnings:
1st. WARNING at lib/percpu-recount.c:309 percpu_ref_kill_and_confirm called more than once
2nd. WARNING at lib/percpu-refcount.c:331
But the most unpleasant effect is a hang of a blk_mq_freeze_queue_wait(), which waits for a zero of a q_usage_counter, which never happens because percpu-ref was reinited (instead of being killed) and stays in PERCPU state forever.
The simplified sequence above can be reproduced on shared tags, when queue A is going to die meanwhile another queue B is in init state and is trying to freeze the queue A, which shares the same tags set:
CPU#0 CPU#1 ------------------------------- ------------------------------------ q1 = blk_mq_init_queue(shared_tags)
q2 = blk_mq_init_queue(shared_tags): blk_mq_add_queue_tag_set(shared_tags): blk_mq_update_tag_set_depth(shared_tags): blk_mq_freeze_queue(q1) blk_cleanup_queue(q1) ... blk_mq_freeze_queue(q1) <<<->>> blk_mq_unfreeze_queue(q1)
[1] Message id: 1443287365-4244-7-git-send-email-akinobu.mita@gmail.com [2] Message id: 1443563240-29306-6-git-send-email-tj@kernel.org
Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com> Cc: Akinobu Mita <akinobu.mita@gmail.com> Cc: Tejun Heo <tj@kernel.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: Christoph Hellwig <hch@lst.de> Cc: linux-block@vger.kernel.org Cc: linux-kernel@vger.kernel.org --- v2: - forgotten hunk from local repo - minor tweaks in the commit message
block/blk-core.c | 3 ++- block/blk-mq.c | 22 +++++++++++----------- include/linux/blkdev.h | 7 ++++++- 3 files changed, 19 insertions(+), 13 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c index ef78848..4fd27e9 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -658,7 +658,7 @@ int blk_queue_enter(struct request_queue *q, gfp_t gfp) return -EBUSY; ret = wait_event_interruptible(q->mq_freeze_wq, - !atomic_read(&q->mq_freeze_depth) || + !q->mq_freeze_depth || blk_queue_dying(q)); if (blk_queue_dying(q)) return -ENODEV; @@ -740,6 +740,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id) __set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags); init_waitqueue_head(&q->mq_freeze_wq); + mutex_init(&q->mq_freeze_lock); /* * Init percpu_ref in atomic mode so that it's faster to shutdown. diff --git a/block/blk-mq.c b/block/blk-mq.c index 6d6f8fe..1f3e81b 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -80,13 +80,13 @@ static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx, void blk_mq_freeze_queue_start(struct request_queue *q) { - int freeze_depth; - - freeze_depth = atomic_inc_return(&q->mq_freeze_depth); - if (freeze_depth == 1) { + mutex_lock(&q->mq_freeze_lock); + if (++q->mq_freeze_depth == 1) { percpu_ref_kill(&q->q_usage_counter); + mutex_unlock(&q->mq_freeze_lock); blk_mq_run_hw_queues(q, false); - } + } else + mutex_unlock(&q->mq_freeze_lock); } EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_start); @@ -124,14 +124,14 @@ EXPORT_SYMBOL_GPL(blk_mq_freeze_queue); void blk_mq_unfreeze_queue(struct request_queue *q) { - int freeze_depth; - - freeze_depth = atomic_dec_return(&q->mq_freeze_depth); - WARN_ON_ONCE(freeze_depth < 0); - if (!freeze_depth) { + mutex_lock(&q->mq_freeze_lock); + q->mq_freeze_depth--; + WARN_ON_ONCE(q->mq_freeze_depth < 0); + if (!q->mq_freeze_depth) { percpu_ref_reinit(&q->q_usage_counter); wake_up_all(&q->mq_freeze_wq); } + mutex_unlock(&q->mq_freeze_lock); } EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue); @@ -2105,7 +2105,7 @@ void blk_mq_free_queue(struct request_queue *q) static void blk_mq_queue_reinit(struct request_queue *q, const struct cpumask *online_mask) { - WARN_ON_ONCE(!atomic_read(&q->mq_freeze_depth)); + WARN_ON_ONCE(!q->mq_freeze_depth); blk_mq_sysfs_unregister(q); diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index f6ff9d1..d692c16 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -445,7 +445,7 @@ struct request_queue { struct mutex sysfs_lock; int bypass_depth; - atomic_t mq_freeze_depth; + int mq_freeze_depth; #if defined(CONFIG_BLK_DEV_BSG) bsg_job_fn *bsg_job_fn; @@ -459,6 +459,11 @@ struct request_queue { #endif struct rcu_head rcu_head; wait_queue_head_t mq_freeze_wq; + /* + * Protect concurrent access to q_usage_counter by + * percpu_ref_kill() and percpu_ref_reinit(). + */ + struct mutex mq_freeze_lock; struct percpu_ref q_usage_counter; struct list_head all_q_node; -- 2.9.0
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