On 5/20/19 9:25 PM, Bob Liu wrote: > 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@xxxxxxxxx > [2] Message id: 1443563240-29306-6-git-send-email-tj@xxxxxxxxxx > [3] https://patchwork.kernel.org/patch/9268199/ Applied, thanks. -- Jens Axboe
