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    SubjectRe: [PATCH 3/3] aio-use-cancellation-list-lazily-fix
    On Fri, 25 Jan 2013 21:30:32 +0800
    Hillf Danton <dhillf@gmail.com> wrote:

    > On Fri, Jan 25, 2013 at 5:43 AM, Kent Overstreet <koverstreet@google.com> wrote:
    > > The cancellation changes were fubar - we can't cancel a kiocb if it
    > > doesn't actually have a cancellation callback.
    > >
    > > The use of xchg() in aio_complete() was right - there we're marking the
    > > kiocb as completed - but we need to use cmpxchg() in kiocb_cancel() - a
    > > lock isn't sufficient since we're synchronizing with aio_complete()
    > > which isn't taking any locks.
    > >
    > > static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb,
    > > struct io_event *res)
    > > {
    > > - kiocb_cancel_fn *cancel;
    > > + kiocb_cancel_fn *old, *cancel;
    > > int ret = -EINVAL;
    > >
    > > - cancel = xchg(&kiocb->ki_cancel, KIOCB_CANCELLED);
    > > - if (!cancel || cancel == KIOCB_CANCELLED)
    > > - return ret;
    > > + /*
    > > + * Don't want to set kiocb->ki_cancel = KIOCB_CANCELLED unless it
    > > + * actually has a cancel function, hence the cmpxchg()
    > > + */
    > > +
    > > + cancel = ACCESS_ONCE(kiocb->ki_cancel);
    > > + do {
    > > + if (!cancel || cancel == KIOCB_CANCELLED)
    > > + return ret;
    > > +
    > > + BUG();
    >
    > Hmm, what is trapped?
    >
    > > + old = cancel;
    > > + cancel = cmpxchg(&kiocb->ki_cancel, old, KIOCB_CANCELLED);
    > > + } while (cancel != old);

    erk, I missed that. What earthly sense is there in putting a BUG() in
    that place.

    I think I'll delete it and pretend I never saw it :(




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