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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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