• Akinobu Mita's avatar
    blk-mq: fix deadlock when reading cpu_list · 60de074b
    Akinobu Mita authored
    CPU hotplug handling for blk-mq (blk_mq_queue_reinit) acquires
    all_q_mutex in blk_mq_queue_reinit_notify() and then removes sysfs
    entries by blk_mq_sysfs_unregister().  Removing sysfs entry needs to
    be blocked until the active reference of the kernfs_node to be zero.
    
    On the other hand, reading blk_mq_hw_sysfs_cpu sysfs entry (e.g.
    /sys/block/nullb0/mq/0/cpu_list) acquires all_q_mutex in
    blk_mq_hw_sysfs_cpus_show().
    
    If these happen at the same time, a deadlock can happen.  Because one
    can wait for the active reference to be zero with holding all_q_mutex,
    and the other tries to acquire all_q_mutex with holding the active
    reference.
    
    The reason that all_q_mutex is acquired in blk_mq_hw_sysfs_cpus_show()
    is to avoid reading an imcomplete hctx->cpumask.  Since reading sysfs
    entry for blk-mq needs to acquire q->sysfs_lock, we can avoid deadlock
    and reading an imcomplete hctx->cpumask by protecting q->sysfs_lock
    while hctx->cpumask is being updated.
    Signed-off-by: default avatarAkinobu Mita <akinobu.mita@gmail.com>
    Reviewed-by: default avatarMing Lei <tom.leiming@gmail.com>
    Cc: Ming Lei <tom.leiming@gmail.com>
    Cc: Wanpeng Li <wanpeng.li@hotmail.com>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
    60de074b
blk-mq-sysfs.c 11 KB