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

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Message-ID: <20100430180247.GA2482@linux.vnet.ibm.com>
Date:	Fri, 30 Apr 2010 11:02:47 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Valdis.Kletnieks@...edu
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>, 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, dhowells@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH tip/core/urgent] fix several lockdep splats, allow
 multiple splats

On Fri, Apr 30, 2010 at 12:16:45PM -0400, Valdis.Kletnieks@...edu wrote:
> On Fri, 30 Apr 2010 11:33:34 EDT, Mathieu Desnoyers said:
> 
> > 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.
> 
> Yeah, that would rock for development kernels - playing whack-a-mole with
> a half-dozen new lockdep whinges can easily stretch out for quite some time.

The RCU-lockdep splats are a bit different in nature than the
deadlock-related splats that lockdep normally prints.  The RCU-lockdep
splats are transient in nature, and it is easy to apply WARN_ON_ONCE().
In contrast, if you permit multiple deadlock-related lockdep splats,
you tend to get lots of warnings about the same deadlock cycle.

So how about an additional kernel configuration variable, default
disabled, perhaps named CONFIG_PROVE_RCU_MULTIPLE, that allows a
single boot to see multiple messages?  Unlike the dyntick-idle
WARN_ON()s that generated multi-gigabyte console logs in a great
hurry, I haven't yet seen excessive quantities of RCU-lockdep splats,
so I don't see the need for an integer limit.

Thoughts?

							Thanx, Paul
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