• Bob Liu's avatar
    blk-mq: fix hang caused by freeze/unfreeze sequence · 7996a8b5
    Bob Liu authored
    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: default avatarHannes Reinecke <hare@suse.com>
    Reviewed-by: default avatarMing Lei <ming.lei@redhat.com>
    Reviewed-by: default avatarBart Van Assche <bvanassche@acm.org>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarRoman Pen <roman.penyaev@profitbricks.com>
    Signed-off-by: default avatarBob Liu <bob.liu@oracle.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    7996a8b5