Subject: Re: rtnl_mutex deadlock?

From: Daniel Borkmann <[email-masked]>
Date: Thu, 06 Aug 2015 16:50:39 +0200

On 08/06/2015 02:30 AM, Herbert Xu wrote:
> On Wed, Aug 05, 2015 at 08:59:07PM +0200, Daniel Borkmann wrote:
>>
>> Here's a theory and patch below. Herbert, Thomas, does this make any
>> sense to you resp. sound plausible? ;)
>
> It's certainly possible.  Whether it's plausible I'm not so sure.
> The netlink hashtable is unlimited in size.  So it should always
> be expanding, not rehashing.  The bug you found should only affect
> rehashing.
>
>> I'm not quite sure what's best to return from here, i.e. whether we
>> propagate -ENOMEM or instead retry over and over again hoping that the
>> rehashing completed (and no new rehashing started in the mean time) ...
>
> Please use something other than ENOMEM as it is already heavily
> used in this context.  Perhaps EOVERFLOW?

Okay, I'll do that.

> We should probably add a WARN_ON_ONCE in rhashtable_insert_rehash
> since two concurrent rehashings indicates something is going
> seriously wrong.

So, if I didn't miss anything, it looks like the following could have
happened: the worker thread, that is rht_deferred_worker(), itself could
trigger the first rehashing, e.g. after shrinking or expanding (or also
in case none of both happen).

Then, in __rhashtable_insert_fast(), I could trigger an -EBUSY when I'm
really unlucky and exceed the ht->elasticity limit of 16. I would then
end up in rhashtable_insert_rehash() to find out there's already one
ongoing and thus, I'm getting -EBUSY via __netlink_insert().

Perhaps that is what could have happened? Seems rare though, but it was
also only seen rarely so far ...