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

Paul E. McKenney
2009-12-14T01:53:47Z

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?

Thanx, Paul