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


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