netdev - Re: rtnl_mutex deadlock?

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Date:	Fri, 07 Aug 2015 01:58:15 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Pirko <jiri@...nulli.us>,
	Cong Wang <cwang@...pensource.com>,
	David Miller <davem@...emloft.net>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Thomas Graf <tgraf@...g.ch>, Scott Feldman <sfeldma@...il.com>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: rtnl_mutex deadlock?

On 08/07/2015 01:41 AM, Herbert Xu wrote:
> On Thu, Aug 06, 2015 at 04:50:39PM +0200, Daniel Borkmann wrote:
>>
>> 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().
>
> Right, so the only way you can trigger this is if you hit a chain
> longer than 16 and the number of entries in the table is less than
> 75% the size of the table, as well as there being an existing resize
> or rehash operation.
>
> This should be pretty much impossible.
>
> But if we had a WARN_ON_ONCE there then we'll know for sure.

Looks like we had a WARN_ON() in rhashtable_insert_rehash() before, but
was removed in a87b9ebf1709 ("rhashtable: Do not schedule more than one
rehash if we can't grow further"). Do you want to re-add a WARN_ON_ONCE()?

Thanks,
Daniel
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