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Subject[RESEND PATCH] blk-mq: fix hang caused by freeze/unfreeze sequence
Date
This patch was proposed by Roman Pen[3] years ago.
Recently we hit a bug which is likely caused by the same reason,so rebased his
fix to v5.1 and resend.
Below is almost copied from that patch[3].

------
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
[3] https://patchwork.kernel.org/patch/9268199/

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Signed-off-by: Bob Liu <bob.liu@oracle.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

---
v3:
- rebase to v5.1
v2:
- forgotten hunk from local repo
- minor tweaks in the commit message
---
block/blk-core.c | 3 ++-
block/blk-mq.c | 19 ++++++++++---------
include/linux/blkdev.h | 7 ++++++-
3 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index a55389b..fb97497 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -433,7 +433,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
smp_rmb();

wait_event(q->mq_freeze_wq,
- (atomic_read(&q->mq_freeze_depth) == 0 &&
+ (!q->mq_freeze_depth &&
(pm || (blk_pm_request_resume(q),
!blk_queue_pm_only(q)))) ||
blk_queue_dying(q));
@@ -523,6 +523,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
spin_lock_init(&q->queue_lock);

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 a935483..373af60 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -143,13 +143,14 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,

void blk_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);
if (queue_is_mq(q))
blk_mq_run_hw_queues(q, false);
+ } else {
+ mutex_unlock(&q->mq_freeze_lock);
}
}
EXPORT_SYMBOL_GPL(blk_freeze_queue_start);
@@ -198,14 +199,14 @@ 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_resurrect(&q->q_usage_counter);
wake_up_all(&q->mq_freeze_wq);
}
+ mutex_unlock(&q->mq_freeze_lock);
}
EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5c58a3b..64f7683 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -535,7 +535,7 @@ struct request_queue {

struct mutex sysfs_lock;

- atomic_t mq_freeze_depth;
+ int mq_freeze_depth;

#if defined(CONFIG_BLK_DEV_BSG)
struct bsg_class_device bsg_dev;
@@ -547,6 +547,11 @@ 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;

--
2.9.5
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