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

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Date:	Fri, 30 Apr 2010 11:32:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Valdis.Kletnieks@...edu, 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 02:12:01PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > 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?
> 
> Ideally we don't want to flood the console with thousands of instances
> of the same RCU-lockdep splat (think of a missing read lock on a common
> code path). Therefore I think keeping an integer limit is relevant here.
> I agree that this integer limit could be selected by a CONFIG_ option
> rather than by a kernel parameter, as it will typically only be used on
> development kernels with "kernel hacking" enabled anyway. There is not
> much point in bloating the kernel code with an extra debug-only kernel
> parameter parsing.

We already limit via WARN_ON_ONCE(), and there are fewer than 500 lines
of code in the kernel that can give RCU lockdep splats, so I really believe
that we are OK without an overall limit for the foreseeable future.

							Thanx, Paul
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