linux-kernel - Re: [patch 0/9] Fix various __task_cred related invalid RCU assumptions

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Date:	Mon, 14 Dec 2009 06:16:07 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...sign.ru>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 0/9] Fix various __task_cred related invalid RCU
	assumptions

On Mon, Dec 14, 2009 at 11:17:39AM +0100, Peter Zijlstra wrote:
> On Sun, 2009-12-13 at 17:53 -0800, Paul E. McKenney wrote:
> > On Sun, Dec 13, 2009 at 07:56:17PM +0100, Peter Zijlstra wrote:
> > > On Wed, 2009-12-09 at 21:34 -0800, Paul E. McKenney wrote:
> > > 
> > > > Ah -- I have a related lockdep question.  Is there a primitive that says
> > > > whether or not the current task holds at least one lock of any type?
> > > > If so, I would like to make rcu_dereference() do at least a little crude
> > > > checking for this problem.
> > > 
> > > Hmm, no, but that's not hard to do, however I actually implemented
> > > something like that for RCU a long while ago and that gives a metric TON
> > > of false positives due to things like the radix tree which are RCU-safe
> > > but are not required to be used with RCU.
> > 
> > Understood -- my current guess is that there needs to be a way to tag
> > a variant of the rcu_dereference() API with the conditions that must be
> > met, for example, either in an rcu-sched read-side critical section or
> > holding a specific type of lock.
> > 
> > This does make it a little harder to retroactively add checking to
> > existing calls to rcu_dereference(), but should allow a good balance
> > between false positives and false negatives going forward.
> > 
> > Seem reasonable, or am I still missing something?
> 
> The only concern is drowning in rcu_dereference() annotations. But I
> guess that is unavoidable.

So far, I haven't been able to think of anything better.  :-/

> I think you can use lock_is_held(&rcu_lock_map), except you need to deal
> with the !debug_locks case, because lockdep stops once debug_locks
> becomes false, which means lock_is_held() will return rubbish.

OK, so I need to do something like the following, then?

	debug_locks ? lock_is_held(&rcu_lock_map) : 1

							Thanx, Paul
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