linux-kernel - Re: [PATCH tip/core/urgent] fix several lockdep splats, allow multiple splats

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Message-ID: <20100430153334.GA19672@Krystal>
Date:	Fri, 30 Apr 2010 11:33:34 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH tip/core/urgent] fix several lockdep splats, allow
	multiple splats

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Paul E. McKenney <paulmck@...ux.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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