linux-kernel - Re: [PATCH 3/3] aio-use-cancellation-list-lazily-fix

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Message-ID: <20130128173733.GV26407@google.com>
Date:	Mon, 28 Jan 2013 09:37:33 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Hillf Danton <dhillf@...il.com>, Valdis.Kletnieks@...edu,
	bcrl@...ck.org, zab@...bo.net, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/3] aio-use-cancellation-list-lazily-fix

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@...il.com> wrote:
> 
> > On Fri, Jan 25, 2013 at 5:43 AM, Kent Overstreet <koverstreet@...gle.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 :(

Argh. Yeah, sorry about that. Put that in there when I was trying to
track down the other bugs :(
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