netdev - Re: [PATCH 1/4] [RFC] net: Explicitly initialize u64_stats

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Date:	Thu, 26 Sep 2013 12:39:04 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Mirko Lindner <mlindner@...vell.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Roger Luethi <rl@...lgate.ch>,
	Patrick McHardy <kaber@...sh.net>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Wensong Zhang <wensong@...ux-vs.org>,
	Simon Horman <horms@...ge.net.au>,
	Julian Anastasov <ja@....bg>, Jesse Gross <jesse@...ira.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 1/4] [RFC] net: Explicitly initialize u64_stats_sync structures
 for lockdep

On 09/26/2013 12:34 PM, John Stultz wrote:
> On 09/26/2013 12:26 PM, Eric Dumazet wrote:
>> On Thu, 2013-09-26 at 11:34 -0700, John Stultz wrote:
>>> In order to enable lockdep on seqcount/seqlock structures, we
>>> must explicitly initialize any locks.
>>>
>>> diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
>>> index 8da8c4e..c450e11 100644
>>> --- a/include/linux/u64_stats_sync.h
>>> +++ b/include/linux/u64_stats_sync.h
>>> @@ -67,6 +67,13 @@ struct u64_stats_sync {
>>>  #endif
>>>  };
>>>  
>>> +
>>> +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
>>> +#define u64_stats_init(syncp)	seqcount_init(syncp.seq)
>>> +#else
>>> +#define u64_stats_init(syncp)
>>> +#endif
>>> +
>> I would prefer a function.
>>
>> static inline void u64_stats_init(struct u64_stats_sync *syncp)
>> {
>> #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
>> 	seqcount_init(&syncp->seq);
>> #endif
>> }
> No objection, though I worry that will obscure the lockdep key name,
> causing all the u64_stat_sync locks to appear the same.
>
> I'll give it a quick shot to see if the compiler inlines it well enough
> to preserve the name.
Yea, unfortunately this causes the reports to all look like:
[   10.370105]  Possible unsafe locking scenario:
[   10.370105]
[   10.370105]        CPU0
[   10.370105]        ----
[   10.370105]   lock(&syncp->seq#5);
[   10.370105]   <Interrupt>
[   10.370105]     lock(&syncp->seq#5);
[   10.370105]
[   10.370105]  *** DEADLOCK ***


Rather then:
[   12.150105]  Possible unsafe locking scenario:
[   12.150105]
[   12.150105]        CPU0
[   12.150105]        ----
[   12.150105]   lock(&af_inet6_stats->syncp.seq);
[   12.150105]   <Interrupt>
[   12.150105]     lock(&af_inet6_stats->syncp.seq);
[   12.150105]
[   12.150105]  *** DEADLOCK ***


So I suspect using the macro is the best approach here. Is that ok?

thanks
-john
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