Date: Fri, 30 Apr 2010 11:33:34 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
To: Ingo Molnar <mingo@elte.hu>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       linux-kernel@vger.kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
       akpm@linux-foundation.org, josh@joshtriplett.org, dvhltc@us.ibm.com,
       niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
       rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com,
       eric.dumazet@gmail.com
Subject: Re: [PATCH tip/core/urgent] fix several lockdep splats, allow
	multiple splats
Message-ID: <20100430153334.GA19672@Krystal>
References: <20100421200150.GA3818@linux.vnet.ibm.com> <20100430100749.GC14202@elte.hu>
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* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > Hello!
> > 
> > This patchset contains four RCU lockdep splat fixes, courtesy of David 
> > Howells, Peter Zijlstra, and Trond Myklebust, [...]
> 
> I've applied #1 and #2 - but shouldnt #4 and #5 go via the NFS tree?
> 
> > [...] as well as an enhancement by Lai Jiangshan that permits collecting 
> > more than one RCU lockdep splat per boot.
> 
> Hm, this #3 patch i disagree with quite fundamentally: one of the big virtues 
> of lockdep is that it complains only once and then shuts up and lets the 
> system work. It allows distro debug kernels to have lockdep enabled, etc.
> 
> One bugreport per bootup per user is the most we can expect really. Not 
> disabling it risks getting a stream of repeat messages, annoyed testers and 
> gives us _less_ bugreports in the end.
> 
> Also, often the _first_ warning is the most reliable one - sometimes there's 
> interactions, and the first bug causing a second warning as well, etc. So 
> reporting just the highest-quality (i.e. first) issue we detect is the best 
> approach.

I recommend creating a kernel command line parameter that would tweak
the number of messages printed by lockdep. The default would indeed by 1
message, but people in a debugging marathon can specify a larger value
so they won't have to reboot between each individual lockdep error.

Thanks,

Mathieu

> 
> Thanks,
> 
> 	Ingo

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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