netdev - Re: suspicious RCU usage (netlink/rhashtable)

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Message-ID: <20151222214734.GB13231@codemonkey.org.uk>
Date:	Tue, 22 Dec 2015 16:47:34 -0500
From:	Dave Jones <davej@...emonkey.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	kraigatgoog@...il.com, netdev@...r.kernel.org,
	herbert@...dor.apana.org.au
Subject: Re: suspicious RCU usage (netlink/rhashtable)

On Tue, Dec 22, 2015 at 04:42:25PM -0500, David Miller wrote:
 > From: Craig Gallek <kraigatgoog@...il.com>
 > Date: Tue, 22 Dec 2015 16:38:32 -0500
 > 
 > > On Tue, Dec 22, 2015 at 4:28 PM, David Miller <davem@...emloft.net> wrote:
 > >> From: Craig Gallek <kraigatgoog@...il.com>
 > >> Date: Tue, 22 Dec 2015 15:51:19 -0500
 > >>
 > >>> I was actually just looking at this as well (though a slightly
 > >>> different stack).  The issue is with: c6ff5268293e rhashtable: Fix
 > >>> walker list corruption
 > >>>
 > >>> It changed the lock acquired in rhashtable_walk_init to use the new
 > >>> spinlock, but the rht_dereference macro expects the mutex.  I was
 > >>> still trying to track down which repository this change came in
 > >>> through, though...
 > >>
 > >> Both cam via my networking tree.
 > > Simple fix is below.  Though, I don't understand the history of the
 > > multiple locks in this structure to be sure it's correct.  I'll send
 > > it as a formal patch.  Please reject if it's not the right approach.
 > > 
 > > diff --git a/lib/rhashtable.c b/lib/rhashtable.c
 > > index 1c149e9..cc80870 100644
 > > --- a/lib/rhashtable.c
 > > +++ b/lib/rhashtable.c
 > > @@ -516,7 +516,8 @@ int rhashtable_walk_init(struct rhashtable *ht,
 > > struct rhashtable_iter *iter)
 > >                 return -ENOMEM;
 > > 
 > >         spin_lock(&ht->lock);
 > > -       iter->walker->tbl = rht_dereference(ht->tbl, ht);
 > > +       iter->walker->tbl =
 > > +               rcu_dereference_protected(ht->tbl, lockdep_is_held(&ht->lock));
 > >         list_add(&iter->walker->list, &iter->walker->tbl->walkers);
 > >         spin_unlock(&ht->lock);
 > 
 > How can this be the "fix"?  That's exactly what's in the tree.

I should have made clear, this is Linus' tree I'm hitting this on,
which matches what Craig posted.

	Dave
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