On Fri, Jan 25, 2013 at 03:12:51PM -0800, Andrew Morton wrote:
> On Fri, 25 Jan 2013 21:30:32 +0800
> Hillf Danton <dhillf@xxxxxxxxx> wrote:
>
> > On Fri, Jan 25, 2013 at 5:43 AM, Kent Overstreet <koverstreet@xxxxxxxxxx> 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 :(
Argh. Yeah, sorry about that. Put that in there when I was trying to
track down the other bugs :(
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