On Thu, Apr 25, 2019 at 9:21 PM Bob Liu <bob.liu@xxxxxxxxxx> wrote: > > The following is a description of a hang in blk_mq_freeze_queue_wait(). > The hang happens on attempting 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@xxxxxxxxx > [2] Message id: 1443563240-29306-6-git-send-email-tj@xxxxxxxxxx > [3] https://patchwork.kernel.org/patch/9268199/ > > Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx> > --- > v4: > - Update commit log > 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; Looks fine, Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> Thanks, Ming Lei
