diff options
author | Bob Liu <bob.liu@oracle.com> | 2019-05-21 06:25:55 +0300 |
---|---|---|
committer | Jens Axboe <axboe@kernel.dk> | 2019-05-23 19:25:26 +0300 |
commit | 7996a8b5511a72465b0b286763c2d8f412b8874a (patch) | |
tree | 8924f9a161678ae643b08134a7e12e77adb55cce /block/blk-mq.c | |
parent | 6869875fbc04042ad01654591da60862706e86e3 (diff) |
blk-mq: fix hang caused by freeze/unfreeze sequence
The following is a description of a hang in blk_mq_freeze_queue_wait().
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_resurrect() and
percpu_ref_kill() and as a result those two can be swapped:
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):
list_for_each_entry()
blk_mq_freeze_queue(q1)
> percpu_ref_kill()
> blk_mq_freeze_queue_wait()
blk_cleanup_queue(q1)
blk_mq_freeze_queue(q1)
> percpu_ref_kill()
^^^^^^ freeze_depth can't guarantee the order
blk_mq_unfreeze_queue()
> percpu_ref_resurrect()
> blk_mq_freeze_queue_wait()
^^^^^^ Hang here!!!!
This wrong sequence raises kernel warning:
percpu_ref_kill_and_confirm called more than once on blk_queue_usage_counter_release!
WARNING: CPU: 0 PID: 11854 at lib/percpu-refcount.c:336 percpu_ref_kill_and_confirm+0x99/0xb0
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.
How to reproduce:
- "insmod null_blk.ko shared_tags=1 nr_devices=0 queue_mode=2"
- cpu0: python Script.py 0; taskset the corresponding process running on cpu0
- cpu1: python Script.py 1; taskset the corresponding process running on cpu1
Script.py:
------
#!/usr/bin/python3
import os
import sys
while True:
on = "echo 1 > /sys/kernel/config/nullb/%s/power" % sys.argv[1]
off = "echo 0 > /sys/kernel/config/nullb/%s/power" % sys.argv[1]
os.system(on)
os.system(off)
------
This bug was first reported and fixed by Roman, previous discussion:
[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/
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/blk-mq.c')
-rw-r--r-- | block/blk-mq.c | 19 |
1 files changed, 10 insertions, 9 deletions
diff --git a/block/blk-mq.c b/block/blk-mq.c index 08a6248d8536..32b8ad3d341b 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -144,13 +144,14 @@ 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); @@ -199,14 +200,14 @@ 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); |