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authorBob Liu <bob.liu@oracle.com>2019-05-21 06:25:55 +0300
committerJens Axboe <axboe@kernel.dk>2019-05-23 19:25:26 +0300
commit7996a8b5511a72465b0b286763c2d8f412b8874a (patch)
tree8924f9a161678ae643b08134a7e12e77adb55cce /block/blk-mq.c
parent6869875fbc04042ad01654591da60862706e86e3 (diff)
downloadlinux-7996a8b5511a72465b0b286763c2d8f412b8874a.tar.xz
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.c19
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);