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Subject: Re: [patch 0/9] Fix various __task_cred related invalid RCU
 assumptions
From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@linux.vnet.ibm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
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       Andrew Morton <akpm@linux-foundation.org>
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Date: Mon, 14 Dec 2009 11:17:39 +0100
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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.

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.