netdev - Re: [PATCH] inetpeer: fix a race in inetpeer_gc

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Message-ID: <20120605124257.GE27795@secunet.com>
Date:	Tue, 5 Jun 2012 14:42:57 +0200
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] inetpeer: fix a race in inetpeer_gc_worker()

On Tue, Jun 05, 2012 at 02:19:12PM +0200, Eric Dumazet wrote:
> On Tue, 2012-06-05 at 13:56 +0200, Steffen Klassert wrote:
> > On Tue, Jun 05, 2012 at 11:28:27AM +0200, Eric Dumazet wrote:
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > 
> > > commit 5faa5df1fa2024 (inetpeer: Invalidate the inetpeer tree along with
> > > the routing cache) added a race :
> > > 
> > > Before freeing an inetpeer, we must respect a RCU grace period, and make
> > > sure no user will attempt to increase refcnt.
> > > 
> > 
> > As already mentioned in the other mail. In this case, I think
> > we can just delete the inetpeer once the refcount got zero.
> > 
> 
> Nope, a concurrent lookup can find an entry about to be freed.

Hm, I agree that we need rcu protection when we remove single entries
from an inetpeer tree. But in this case we invalidate the entire tree.

The first lookup after inetpeer_invalidate_tree() was invoked should
find an empty tree, base->root initialized to peer_avl_empty_rcu.

Maybe I'm wrong, but I don't see how a lookup should find such an
old invalidated tree.

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