Messages in this thread | | | Date | Fri, 25 Jan 2013 15:12:51 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 3/3] aio-use-cancellation-list-lazily-fix |
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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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