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

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Message-ID: <CAEfhGizBuy79qvUdOFtbpUHg-2NPb3=Lsms8Mhg4txQH6-CWyA@mail.gmail.com>
Date:	Tue, 22 Dec 2015 16:46:38 -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 Xu <herbert@...dor.apana.org.au>
Subject: Re: suspicious RCU usage (netlink/rhashtable)

On Tue, Dec 22, 2015 at 4:42 PM, David Miller <davem@...emloft.net> 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.
Ah, you're right, this fix was submitted to next in 179ccc0a7364 but
hasn't made it into net-next yet.
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