Re: [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 [off-list ref]
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 ;-/