• Linus Torvalds's avatar
    block: don't use un-ordered __set_current_state(TASK_UNINTERRUPTIBLE) · 1ac5cd49
    Linus Torvalds authored
    This mostly reverts commit 849a3700 ("block: avoid ordered task
    state change for polled IO").  It was wrongly claiming that the ordering
    wasn't necessary.  The memory barrier _is_ necessary.
    
    If something is truly polling and not going to sleep, it's the whole
    state setting that is unnecessary, not the memory barrier.  Whenever you
    set your state to a sleeping state, you absolutely need the memory
    barrier.
    
    Note that sometimes the memory barrier can be elsewhere.  For example,
    the ordering might be provided by an external lock, or by setting the
    process state to sleeping before adding yourself to the wait queue list
    that is used for waking up (where the wait queue lock itself will
    guarantee that any wakeup will correctly see the sleeping state).
    
    But none of those cases were true here.
    
    NOTE! Some of the polling paths may indeed be able to drop the state
    setting entirely, at which point the memory barrier also goes away.
    
    (Also note that this doesn't revert the TASK_RUNNING cases: there is no
    race between a wakeup and setting the process state to TASK_RUNNING,
    since the end result doesn't depend on ordering).
    
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Christoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    1ac5cd49
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