Re: [Lse-tech] Re: [PATCH 0/7]: Fix for unsafe notifier chain

Andrew Morton
2005-11-27T19:57:16Z

Keith Owens <kaos@sgi.com> wrote:
>
> On Sun, 27 Nov 2005 18:27:25 +0100,
> Andi Kleen <ak@suse.de> wrote:
> >On Mon, Nov 28, 2005 at 02:59:05AM +1100, Keith Owens wrote:
> >> On Sun, 27 Nov 2005 14:47:36 +0100,
> >> Andi Kleen <ak@suse.de> wrote:
> >> >akpm wrote
> >> >> - Introduce a new notifier API which is wholly unlocked
> >> >
> >> >The old notifiers were already wholly unlocked.

The register and unregister functions take a write_lock on notifier_lock.
notifier_call_chain() runs unlocked.

> So it wouldn't
> >> >even need any changes. Just additional locks everywhere.
> >>
> >> Wrong.
> >
> >Did you actually read what I wrote?
>
> Of course I did.

No you didn't!

> The whole point is that _ALL_ of the existing
> notifier chain callback code is racy[*].

yup.

> Saying that the code can be
> left without any changes is simply ignoring the existing races. They
> _ALL_ need to be fixed.

We're saying that kernel/sys.c:notifier_lock should be removed and that all
callers of notifier_chain_register(), notifier_chain_unregister() and
notifier_call_chain() should be changed to define and use their own lock.

So the _callers_ get to decide whether they're going to use down(),
spin_lock(), down_read(), read_lock(), preempt_disable(), local_irq_disable()
or whatever.

Furthermore we should alter notifier_call_chain() so that a callback may
safely perform notifier_chain_unregister() - that's sane and easy enough.