linux-kernel - Re: [PATCH tip/core/rcu 01/48] rcu: optionally leave lockdep enabled after RCU lockdep splat

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Date:	Wed, 5 May 2010 16:05:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, 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/rcu 01/48] rcu: optionally leave lockdep
	enabled after RCU lockdep splat

On Wed, May 05, 2010 at 06:46:41PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > From: Lai Jiangshan <laijs@...fujitsu.com>
> > 
> > There is no need to disable lockdep after an RCU lockdep splat,
> > so remove the debug_lockdeps_off() from lockdep_rcu_dereference().
> > To avoid repeated lockdep splats, use a static variable in the inlined
> > rcu_dereference_check() and rcu_dereference_protected() macros so that
> > a given instance splats only once, but so that multiple instances can
> > be detected per boot.
> > 
> > This is controlled by a new config variable CONFIG_PROVE_RCU_REPEATEDLY,
> > which is disabled by default.  This provides the normal lockdep behavior
> > by default, but permits people who want to find multiple RCU-lockdep
> > splats per boot to easily do so.
> 
> I'll play the devil's advocate here. (just because that's so much fun)
> ;-)
> 
> If we look at:
> 
> include/linux/debug_locks.h:
> 
> static inline int __debug_locks_off(void)
> {
>         return xchg(&debug_locks, 0);
> }
> 
> We see that all code following a call to "debug_locks_off()" can assume
> that it cannot possibly run concurrently with other code following
> "debug_locks_off()". Now, I'm not saying that the code we currently have
> will necessarily break, but I think it is worth asking if there is any
> assumption in lockdep (or RCU lockdep more specifically) about mutual
> exclusion after debug_locks_off() ?
> 
> Because if there is, then this patch is definitely breaking something by
> not protecting lockdep against multiple concurrent executions.

So what in lockdep_rcu_dereference() needs to be protected from concurrent
execution?  All that happens beyond that point is a bunch of printk()s,
printing the locks held by this task, and dumping this task's stack.

							Thanx, Paul
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