netdev - Re: [PATCH net] ipv6: sr: fix scheduling in RCU when creating seg6 lwtunnel state

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Message-Id: <20180322.122226.1774912567663468513.davem@davemloft.net>
Date:   Thu, 22 Mar 2018 12:22:26 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     dav.lebrun@...il.com, netdev@...r.kernel.org, dlebrun@...gle.com,
        roopa@...ulusnetworks.com
Subject: Re: [PATCH net] ipv6: sr: fix scheduling in RCU when creating seg6
 lwtunnel state

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 20 Mar 2018 10:20:15 -0700

> 
> 
> On 03/20/2018 10:11 AM, David Lebrun wrote:
>> On 20/03/18 15:07, Eric Dumazet wrote:
>>> This is not the proper fix.
>>>
>>> Control path holds RTNL and can sleeep if needed.
>>>
>>> RCU should be avoided in lwtunnel_build_state()
>>>
>> 
>> +Roopa
>> 
>> In lwtunnel_build_state(), the RCU protects the lwtunnel_encap_ops "ops" which is rcu-dereferenced. Moreover, the lwtunnel_state_alloc() function, which is used in all build_state functions, also uses GFP_ATOMIC, so this seemed a proper fix, or at least proper mitigation.
>> 
>> Do you suggest that the lwtunnel_encap_ops can be protected in a different way, not requiring RCU ?
> 
> Yes, GFP_ATOMIC might be an 'easy fix' for net tree,
> but for the future, GFP_KERNEL allocations make more sense in control path.

Agreed.

I'll apply this to net and queue it up for -stable, but long term making
this able to use GFP_KERNEL properly is the real way to go about it.