[PATCH v2 5/6] blk-mq: fix freeze queue race

[Akinobu Mita <akinobu.mita@gmail.com>]

There are several race conditions while freezing queue.

When unfreezing queue, there is a small window between decrementing
q->mq_freeze_depth to zero and percpu_ref_reinit() call with
q->mq_usage_counter.  If the other calls blk_mq_freeze_queue_start()
in the window, q->mq_freeze_depth is increased from zero to one and
percpu_ref_kill() is called with q->mq_usage_counter which is already
killed.  percpu refcount should be re-initialized before killed again.

Also, there is a race condition while switching to percpu mode.
percpu_ref_switch_to_percpu() and percpu_ref_kill() must not be
executed at the same time as the following scenario is possible:

1. q->mq_usage_counter is initialized in atomic mode.
   (atomic counter: 1)

2. After the disk registration, a process like systemd-udev starts
   accessing the disk, and successfully increases refcount successfully
   by percpu_ref_tryget_live() in blk_mq_queue_enter().
   (atomic counter: 2)

3. In the final stage of initialization, q->mq_usage_counter is being
   switched to percpu mode by percpu_ref_switch_to_percpu() in
   blk_mq_finish_init().  But if CONFIG_PREEMPT_VOLUNTARY is enabled,
   the process is rescheduled in the middle of switching when calling
   wait_event() in __percpu_ref_switch_to_percpu().
   (atomic counter: 2)

4. CPU hotplug handling for blk-mq calls percpu_ref_kill() to freeze
   request queue.  q->mq_usage_counter is decreased and marked as
   DEAD.  Wait until all requests have finished.
   (atomic counter: 1)

5. The process rescheduled in the step 3. is resumed and finishes
   all remaining work in __percpu_ref_switch_to_percpu().
   A bias value is added to atomic counter of q->mq_usage_counter.
   (atomic counter: PERCPU_COUNT_BIAS + 1)

6. A request issed in the step 2. is finished and q->mq_usage_counter
   is decreased by blk_mq_queue_exit().  q->mq_usage_counter is DEAD,
   so atomic counter is decreased and no release handler is called.
   (atomic counter: PERCPU_COUNT_BIAS)

7. CPU hotplug handling in the step 4. will wait forever as
   q->mq_usage_counter will never be zero.

Also, percpu_ref_reinit() and percpu_ref_kill() must not be executed
at the same time.  Because both functions could call
__percpu_ref_switch_to_percpu() which adds the bias value and
initialize percpu counter.

Fix those races by serializing with per-queue mutex.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <tom.leiming@gmail.com>
---
 block/blk-core.c       | 1 +
 block/blk-mq-sysfs.c   | 2 ++
 block/blk-mq.c         | 8 ++++++++
 include/linux/blkdev.h | 6 ++++++
 4 files changed, 17 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index bbf67cd..f3c5ae2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -687,6 +687,7 @@ 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);
 	mutex_init(&q->mq_sysfs_lock);
 
 	if (blkcg_init_queue(q))
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 79a3e8d..8448513 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -413,7 +413,9 @@ static void blk_mq_sysfs_init(struct request_queue *q)
 /* see blk_register_queue() */
 void blk_mq_finish_init(struct request_queue *q)
 {
+	mutex_lock(&q->mq_freeze_lock);
 	percpu_ref_switch_to_percpu(&q->mq_usage_counter);
+	mutex_unlock(&q->mq_freeze_lock);
 }
 
 int blk_mq_register_disk(struct gendisk *disk)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ad07373..f31de35 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -115,11 +115,15 @@ void blk_mq_freeze_queue_start(struct request_queue *q)
 {
 	int freeze_depth;
 
+	mutex_lock(&q->mq_freeze_lock);
+
 	freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
 	if (freeze_depth == 1) {
 		percpu_ref_kill(&q->mq_usage_counter);
 		blk_mq_run_hw_queues(q, false);
 	}
+
+	mutex_unlock(&q->mq_freeze_lock);
 }
 EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_start);
 
@@ -143,12 +147,16 @@ void blk_mq_unfreeze_queue(struct request_queue *q)
 {
 	int freeze_depth;
 
+	mutex_lock(&q->mq_freeze_lock);
+
 	freeze_depth = atomic_dec_return(&q->mq_freeze_depth);
 	WARN_ON_ONCE(freeze_depth < 0);
 	if (!freeze_depth) {
 		percpu_ref_reinit(&q->mq_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 c56f5a6..0bf8bea 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -457,6 +457,12 @@ struct request_queue {
 #endif
 	struct rcu_head		rcu_head;
 	wait_queue_head_t	mq_freeze_wq;
+	/*
+	 * Protect concurrent access to mq_usage_counter by
+	 * percpu_ref_switch_to_percpu(), percpu_ref_kill(), and
+	 * percpu_ref_reinit().
+	 */
+	struct mutex		mq_freeze_lock;
 	struct percpu_ref	mq_usage_counter;
 	struct list_head	all_q_node;
 
-- 
1.9.1