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    SubjectRe: [RFC] RCU and CONFIG_PREEMPT_RT progress
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    On Mon, 2005-05-09 at 18:24 -0700, Paul E. McKenney wrote:

    >
    > Counter-Based Approach
    >
    > The current implementation in Ingo's CONFIG_PREEMPT_RT patch uses a
    > counter-based approach, which seems to work, but which can result in
    > indefinite-duration grace periods. The following are very hazy thoughts
    > on how to get the benefits of this approach, but with short grace periods.
    >
    > 1. The basic trick is to maintain a pair of counters per CPU.
    > There would also be a global boolean variable that would select
    > one or the other of each pair. The rcu_read_lock() primitive
    > would then increment the counter indicated by the boolean
    > corresponding to the CPU that it is currently running on.
    > It would also keep a pointer to that particular counter in
    > the task structure. The rcu_read_unlock() primitive would
    > decrement this counter. (And, yes, you would also have a
    > counter in the task structure so that only the outermost of
    > a set of nested rcu_read_lock()/rcu_read_unlock() pairs would
    > actually increment/decrement the per-CPU counter pairs.)
    >
    > To force a grace period, one would invert the value of the
    > global boolean variable. Once all the counters indicated
    > by the old value of the global boolean variable hit zero,
    > the corresponding set of RCU callbacks can be safely invoked.
    >
    > The big problem with this approach is that a pair of inversions
    > of the global boolean variable could be spaced arbitrarily
    > closely, especially when you consider that the read side code
    > can be preempted. This could cause RCU callbacks to be invoked
    > prematurely, which could greatly reduce the life expectancy
    > of your kernel.

    > Thoughts?
    >

    How about having another boolean indicating the ability to flip the
    selector boolean. This boolean would be set false on an actual flip and
    cleared during a grace period. That way the flips cannot ever interfere
    with one another such that the callbacks would be cleared prematurely.

    --
    Peter Zijlstra <a.p.zijlstra@chello.nl>

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