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

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Message-ID: <CAEfhGiz1FnghHRpcYUVKhWNV=4yjfU-pAsjynKTgwPctsNQ5Gw@mail.gmail.com>
Date:	Tue, 22 Dec 2015 16:38:32 -0500
From:	Craig Gallek <kraigatgoog@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Dave Jones <davej@...emonkey.org.uk>, netdev@...r.kernel.org,
	herbert@...dor.apana.org.au
Subject: Re: suspicious RCU usage (netlink/rhashtable)

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);
--
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