linux-kernel - Re: [PATCH 4/4] [RFC] ipv6: Fix for possible ipv6 seqlock deadlock

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Date:	Fri, 27 Sep 2013 09:00:27 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH 4/4] [RFC] ipv6: Fix for possible ipv6 seqlock deadlock

On 09/26/2013 10:47 PM, Ingo Molnar wrote:
> * John Stultz <john.stultz@...aro.org> wrote:
>
>> On 09/26/2013 12:30 PM, Eric Dumazet wrote:
>>> On Thu, 2013-09-26 at 11:34 -0700, John Stultz wrote:
>>>> While enabling lockdep on seqlocks, I ran accross the warning below
>>>> caused by the ipv6 stats being updated in both irq and non-irq context.
>>>>
>>>> This is a novice attempt to correct the issue, and with this patch
>>>> the warning goes away. Any better solutions would be appreciated!
>>> ...
>>>
>>>>  
>>>>  out_err_release:
>>>> -	if (err == -ENETUNREACH)
>>>> +	if (err == -ENETUNREACH) {
>>>> +		unsigned long flags;
>>>> +		/* some of the stats are locked in irq context */
>>>> +		local_irq_save(flags);
>>>>  		IP6_INC_STATS_BH(net, NULL, IPSTATS_MIB_OUTNOROUTES);
>>>> +		local_irq_restore(flags);
>>>> +	}
>>>>  	dst_release(*dst);
>>>>  	*dst = NULL;
>>>>  	return err;
>>> You better change IP6_INC_STATS_BH() to IP6_INC_STATS()
>>>
>>> On x86 for example, no extra code will be generated (no need to disable
>>> hard or soft irqs)
>> Ah, thanks! Verified it resolves the warning as well, so I've changed my
>> patch to use your suggestion.
> Btw., is this a deadlock that could occur upstream? If yes then adding a 
> Cc: <stable@...nel.org> tag might be warranted.

It seems as such, so I'll add stable on my next submission.

thanks
-john

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