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    SubjectRe: [PATCH 2/8] aio_poll_wake(): don't set ->woken if we ignore the wakeup
    On Thu, Mar 07, 2019 at 12:03:10AM +0000, Al Viro wrote:
    > From: Al Viro <viro@zeniv.linux.org.uk>
    >
    > In case of early wakeups, aio_poll() assumes that aio_poll_complete()
    > has either already happened or is imminent. In that case we do not
    > want to put iocb on the list of cancellables. However, ignored
    > wakeups need to be treated as if wakeup has not happened at all.
    > Trivially fixed by having aio_poll_wake() set ->woken only after
    > it's committed to taking iocb out of the waitqueue.
    >
    > Spotted-by: zhengbin <zhengbin13@huawei.com>
    > Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

    ... and unfortunately it's worse than just that - what both of us
    have missed is that one could have non-specific wakep + schedule_work +
    aio_poll_complete_work() rechecking ->poll(), seeing nothing of
    interest and reinserting into queue. All before vfs_poll() manages
    to return into aio_poll(). The window is harder to hit, but it's
    still there, with exact same "failed to add to cancel list" kind of bug
    if we do hit it ;-/

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