netdev - Re: [PATCH] ipv4: fix the rcu race between free_fib_info and ip_route_output

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Message-ID: <1337759279.14538.201.camel@ymzhang.sh.intel.com>
Date:	Wed, 23 May 2012 15:47:59 +0800
From:	Yanmin Zhang <yanmin_zhang@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, kunx.jiang@...el.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipv4: fix the rcu race between free_fib_info and
 ip_route_output_slow

On Wed, 2012-05-23 at 09:39 +0200, Eric Dumazet wrote:
> On Wed, 2012-05-23 at 15:24 +0800, Yanmin Zhang wrote:
> > On Wed, 2012-05-23 at 09:13 +0200, Eric Dumazet wrote:
> > > On Wed, 2012-05-23 at 08:55 +0200, Eric Dumazet wrote:
> > > 
> > > > Please hold on, I'll send a v2
> > > 
> > > I believe your patch should be fine, if you move back the
> > > fib_info_cnt--;
> > > 
> > > So only do the dev_put() in free_fib_info_rcu().
> > We would do so in a new patch.
> > 
> > > 
> > > No need to clear nh_dev to NULL since we are freeing fi at the end of
> > > function.
> > David suggests to reset it to NULL to detect other potential
> > race conditions.
> > 
> 
> Yes but no.
> 
> Users are in a RCU read lock and we should not change nh_dev to NULL
> while they are running.
> 
> Thats what I tried to do (David message gave me this wrong hint) but it
> came to a dead issue.
> 
> Only after a grace period we can :
> 	dev_put() all involved net_devices
> 	kfree(fi)
> 
> > Besides above suggestions, how do you think about:
> > 
> > fib_create_info=>fib_find_info, but fib_find_info is not protected by
> > fib_info_lock. See the codes:
> > 
> > fib_create_info()
> > {
> > 	...
> > link_it:
> >         ofi = fib_find_info(fi);
> >         if (ofi) {
> >                 fi->fib_dead = 1;
> >                 free_fib_info(fi);
> >                 ofi->fib_treeref++;
> >                 return ofi;
> >         }
> >         fi->fib_treeref++;
> >         atomic_inc(&fi->fib_clntref);
> >         spin_lock_bh(&fib_info_lock);
> > 
> > 	...
> > }
> > 
> > I plan to change it to hold fib_info_lock before calling fib_find_info. Is
> > it ok for you?
> 
> Its not needed we hold RTNL mutex.
Thanks. We would work out a new patch and post it here after running MTBF
testing.


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