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Date: Mon, 8 Aug 2016 13:39:08 +0200
From: Roman Pen <roman.penyaev@...fitbricks.com>
To: unlisted-recipients:; (no To-header on input)
Cc: Roman Pen <roman.penyaev@...fitbricks.com>,
Akinobu Mita <akinobu.mita@...il.com>,
Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
Christoph Hellwig <hch@....de>, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/1] blk-mq: fix hang caused by freeze/unfreeze sequence
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@...il.com
[2] Message id: 1443563240-29306-6-git-send-email-tj@...nel.org
Signed-off-by: Roman Pen <roman.penyaev@...fitbricks.com>
Cc: Akinobu Mita <akinobu.mita@...il.com>
Cc: Tejun Heo <tj@...nel.org>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Christoph Hellwig <hch@....de>
Cc: linux-block@...r.kernel.org
Cc: linux-kernel@...r.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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