Re: [PATCH (regression)] Fragments: fix race between inet_frag_find and inet_frag_secret_rebuild

[Jarek Poplawski <jarkao2@gmail.com>]

On Wed, Jun 25, 2008 at 10:42:56AM +0400, Pavel Emelyanov wrote:
> Jarek Poplawski wrote:
> > Pavel Emelyanov wrote, On 06/24/2008 12:43 PM:
> > 
> >> The problem is that while we work w/o the inet_frags.lock even
> >> read-locked the secret rebuild timer may occur (on another CPU,
> > - since BHs are still disables in the inet_frag_find) and change 
> > 
> > + since BHs are still disabled in the inet_frag_find) and change 
> > 
> >> the rnd seed for ipv4/6 fragments.
> >>
> >> It was caused by my patch fd9e63544cac30a34c951f0ec958038f0529e244
> >> ([INET]: Omit double hash calculations in xxx_frag_intern) late 
> >> in the 2.6.24 kernel, so this should probably be queued to -stable.
> >>
> >> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> >>
> >> ---
> >>
> >> diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
> >> index 4ed429b..0546a0b 100644
> >> --- a/net/ipv4/inet_fragment.c
> >> +++ b/net/ipv4/inet_fragment.c
> >> @@ -192,14 +192,21 @@ EXPORT_SYMBOL(inet_frag_evictor);
> >>  
> >>  static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
> >>  		struct inet_frag_queue *qp_in, struct inet_frags *f,
> >> -		unsigned int hash, void *arg)
> >> +		void *arg)
> >>  {
> >>  	struct inet_frag_queue *qp;
> >>  #ifdef CONFIG_SMP
> >>  	struct hlist_node *n;
> >>  #endif
> >> +	unsigned int hash;
> >>  
> >>  	write_lock(&f->lock);
> >> +	/*
> >> +	 * While we stayed w/o the lock other CPU could update
> >> +	 * the rnd seed, so we need to re-calculate the hash
> >> +	 * chain. Fortunatelly the qp_in can be used to get one.
> >> +	 */
> >> +	hash = f->hashfn(qp_in);
> >>  #ifdef CONFIG_SMP
> >>  	/* With SMP race we have to recheck hash table, because
> >>  	 * such entry could be created on other cpu, while we
> > 
> > Maybe it's a matter of taste: since there is this "#ifdef CONFIG_SMP",
> > and the new comment concerns with "other CPU", why this re-calculation
> > isn't done only for SMP? 
> 
> Because the hash value is required also *outside* this ifdef and adding
> a fancier logic is probably not good for a -rc7 fix.

I'm not sure what fancier logic do you mean: I've thought about simply
leaving the "hash" as functions argument as it is, and only adding this
recalculation under #ifdef CONFIG_SMP, but I can miss something...

Jarek P.