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

     \ /
      Last update: 2009-12-14 11:21    [W:0.020 / U:3.496 seconds]
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