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SubjectRe: [PATCH tip/core/rcu 18/23] rcu: Add random PROVE_RCU_DELAY to grace-period initialization
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On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
>
> 1. CPU 0 completes a grace period, but needs an additional
> grace period, so starts initializing one, initializing all
> the non-leaf rcu_node strcutures and the first leaf rcu_node
> structure. Because CPU 0 is both completing the old grace
> period and starting a new one, it marks the completion of
> the old grace period and the start of the new grace period
> in a single traversal of the rcu_node structures.
>
> Therefore, CPUs corresponding to the first rcu_node structure
> can become aware that the prior grace period has ended, but
> CPUs corresponding to the other rcu_node structures cannot
> yet become aware of this.
>
> 2. CPU 1 passes through a quiescent state, and therefore informs
> the RCU core. Because its leaf rcu_node structure has already
> been initialized, so this CPU's quiescent state is applied to
> the new (and only partially initialized) grace period.
>
> 3. CPU 1 enters an RCU read-side critical section and acquires
> a reference to data item A. Note that this critical section
> will not block the new grace period.
>
> 4. CPU 16 exits dyntick-idle mode. Because it was in dyntick-idle
> mode, some other CPU informed the RCU core of its extended
> quiescent state for the past several grace periods. This means
> that CPU 16 is not yet aware that these grace periods have ended.
>
> 5. CPU 16 on the second leaf rcu_node structure removes data item A
> from its enclosing data structure and passes it to call_rcu(),
> which queues a callback in the RCU_NEXT_TAIL segment of the
> callback queue.
>
> 6. CPU 16 enters the RCU core, possibly because it has taken a
> scheduling-clock interrupt, or alternatively because it has
> more than 10,000 callbacks queued. It notes that the second
> most recent grace period has ended (recall that it cannot yet
> become aware that the most recent grace period has completed),
> and therefore advances its callbacks. The callback for data
> item A is therefore in the RCU_NEXT_READY_TAIL segment of the
> callback queue.
>
> 7. CPU 0 completes initialization of the remaining leaf rcu_node
> structures for the new grace period, including the structure
> corresponding to CPU 16.
>
> 8. CPU 16 again enters the RCU core, again, possibly because it has
> taken a scheduling-clock interrupt, or alternatively because
> it now has more than 10,000 callbacks queued. It notes that
> the most recent grace period has ended, and therefore advances
> its callbacks. The callback for data item A is therefore in
> the RCU_NEXT_TAIL segment of the callback queue.
>
> 9. All CPUs other than CPU 1 pass through quiescent states, so that
> the new grace period completes. Note that CPU 1 is still in
> its RCU read-side critical section, still referencing data item A.
>
> 10. Suppose that CPU 2 is the last CPU to pass through a quiescent
> state for the new grace period, and suppose further that CPU 2
> does not have any callbacks queued. It therefore traverses
> all of the rcu_node structures, marking the new grace period
> as completed, but does not initialize a new grace period.
>
> 11. CPU 16 yet again enters the RCU core, yet again possibly because
> it has taken a scheduling-clock interrupt, or alternatively
> because it now has more than 10,000 callbacks queued. It notes
> that the new grace period has ended, and therefore advances
> its callbacks. The callback for data item A is therefore in
> the RCU_DONE_TAIL segment of the callback queue. This means
> that this callback is now considered ready to be invoked.
>
> 12. CPU 16 invokes the callback, freeing data item A while CPU 1
> is still referencing it.

This is the same scenario as the previous patch (17), right?

However did you find a 12-stage race like that, is that PROMELA goodness
or are you training to beat some chess champion?


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