• Asias He's avatar
    block: Avoid missed wakeup in request waitqueue · 458f27a9
    Asias He authored
    
    
    After hot-unplug a stressed disk, I found that rl->wait[] is not empty
    while rl->count[] is empty and there are theads still sleeping on
    get_request after the queue cleanup. With simple debug code, I found
    there are exactly nr_sleep - nr_wakeup of theads in D state. So there
    are missed wakeup.
    
      $ dmesg | grep nr_sleep
      [   52.917115] ---> nr_sleep=1046, nr_wakeup=873, delta=173
      $ vmstat 1
      1 173  0 712640  24292  96172 0 0  0  0  419  757  0  0  0 100  0
    
    To quote Tejun:
    
      Ah, okay, freed_request() wakes up single waiter with the assumption
      that after the wakeup there will at least be one successful allocation
      which in turn will continue the wakeup chain until the wait list is
      empty - ie. waiter wakeup is dependent on successful request
      allocation happening after each wakeup.  With queue marked dead, any
      woken up waiter fails the allocation path, so the wakeup chaining is
      lost and we're left with hung waiters. What we need is wake_up_all()
      after drain completion.
    
    This patch fixes the missed wakeup by waking up all the theads which
    are sleeping on wait queue after queue drain.
    
    Changes in v2: Drop waitqueue_active() optimization
    Acked-by: default avatarTejun Heo <tj@kernel.org>
    Signed-off-by: default avatarAsias He <asias@redhat.com>
    
    Fixed a bug by me, where stacked devices would oops on calling
    blk_drain_queue() since ->rq.wait[] do not get initialized unless
    it's a full queue setup.
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    458f27a9