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    SubjectRe: [PATCH 07/30] aio: add delayed cancel support
    On Thu, Mar 29, 2018 at 10:33:05PM +0200, Christoph Hellwig wrote:
    > The upcoming aio poll support would like to be able to complete the
    > iocb inline from the cancellation context, but that would cause a
    > double lock of ctx_lock with the current locking scheme. Move the
    > cancelation outside the context lock to avoid this reversal, which
    > suits the existing usb gadgets users just fine as well (in fact
    > both unconditionally disable irqs and thus seem broken without
    > this change).
    >
    > To make this safe aio_complete needs to check if this call should
    > complete the iocb. If it didn't the callers must not release any
    > other resources.

    Uh-oh... What happens to existing users of kiocb_set_cancel_fn() now?
    AFAICS, those guys will *not* get aio_kiocb freed at all in case of
    io_cancel(2). Look: we mark them with AIO_IOCB_CANCELLED and
    call whatever ->ki_cancel() the driver has set. Later the damn
    thing calls ->ki_complete() (i.e. aio_complete_rw()), which calls
    aio_complete(iocb, res, res2, 0) and gets false. Nothing's freed,
    struct file is leaked.

    Frankly, the more I look at that, the less I like what you've done
    with ->ki_cancel() overloading. In regular case it's just accelerating
    the call of ->ki_complete(), which will do freeing. Here you have
    ->ki_cancel() free the damn thing, with the resulting need to play
    silly buggers with locking, freeing logics in aio_complete(), etc.

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