Re: [PATCH] Move inetdev/ifa over to RCU

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

On Sun, Aug 15, 2004 at 07:58:20PM -0700, David S. Miller wrote:
> On Sat, 14 Aug 2004 10:34:28 +1000
> Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> 
> > BTW, it looks like you can remove inetdev_lock altogether.  All its
> > users already assume that they don't race against each other by
> > taking the rtnl lock.
> 
> I think it's needed at least for the:
> 
>       dev->ip_ptr = idev;
>       in_dev_get(idev);

You mean in_dev_hold?

> thing, isn't it?

I don't think so.  But there is a real bug here :)

The lock doesn't really do much since everyone who's holding
it (they're all in devinet.c) already hold the RTNL lock
(the rotten old lock perhaps :)

But the RCU change has created a real bug here.  Imagine this:

CPU0                                    CPU1
inetdev_init
        dev->ip_ptr = idev
                                        in_dev_get
                                                atomic_inc on refcnt

                                        in_dev_put => frees idev
        in_dev_hold(idev) => BUG

So we should reverse the two statements and add an smp_wmb().

Unfortunately the lock doesn't really help you at all.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt