linux-kernel - Re: [PATCH RFC tip/core/rcu 18/18] rcu: add primitives to check for RCU read-side critical sections

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Message-ID: <20091216151848.GC6744@linux.vnet.ibm.com>
Date:	Wed, 16 Dec 2009 07:18:48 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [PATCH RFC tip/core/rcu 18/18] rcu: add primitives to check
	for RCU read-side critical sections

On Wed, Dec 16, 2009 at 11:31:34AM +0100, Peter Zijlstra wrote:
> On Tue, 2009-12-15 at 15:02 -0800, Paul E. McKenney wrote:
> > 
> > Also create rcu_dereference_check(), which takes a second boolean argument
> > into which one puts rcu_read_lock_held() or similar.  For example:
> > 
> >         rcu_dereference_check(gp, rcu_read_lock_held() ||
> >                                   lockdep_is_held(my_lock));
> 
> Ah, so you're going to tackle this the other way around, interesting :-)

Still feeling my way around this one, but for the moment, yes.  ;-)

One potential issue is that for lockdep, avoiding false positives means
erring on the side of the lock -not- being held, while for this approach
to rcu_dereference() checking, avoiding false positives means erring on
the side of the lock being held.

I might need to create a CONFIG_PROVE_LOCKING_RCU to allow shutting off
the more-detailed RCU checking when people want to do partial deadlock
checking, but will see how it goes.

							Thanx, Paul
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