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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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