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SubjectRe: [PATCH tip/core/rcu 17/23] rcu: Fix day-zero grace-period initialization/cleanup race
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On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> The current approach to grace-period initialization is vulnerable to
> extremely low-probabity races. These races stem fro the fact that the
> old grace period is marked completed on the same traversal through the
> rcu_node structure that is marking the start of the new grace period.
> These races can result in too-short grace periods, as shown in the
> following scenario:
>
> 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 completed, but
> CPUs corresponding to the other rcu_node structures will see
> this same prior grace period as still being in progress.
>
> 2. CPU 1 passes through a quiescent state, and therefore informs
> the RCU core. Because its leaf rcu_node structure has already
> been initialized, 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
> started after the beginning of the new grace period, and
> therefore will not block this new grace period.
>
> 4. CPU 16 exits dyntick-idle mode. Because it was in dyntick-idle
> mode, other CPUs 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 past grace periods have ended.
> Assume that CPU 16 corresponds to the second leaf rcu_node
> structure.
>
> 5. CPU 16 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 completed (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_WAIT_TAIL segment of the callback queue.
>
> 9. All CPUs other than CPU 1 pass through quiescent states. Because
> CPU 1 already passed through its quiescent state, 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 wais the last CPU to pass through a quiescent
> state for the new grace period, and suppose further that CPU 2
> did not have any callbacks queued, therefore not needing an
> additional grace period. CPU 2 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 scenario represents a day-zero bug for TREE_RCU. This commit
> therefore ensures that the old grace period is marked completed in
> all leaf rcu_node structures before a new grace period is marked
> started in any of them.


OK, so the above doesn't make it immediately obvious if the described
scenario (glossed the 1-12) is due to the previous patches or was
pre-existing.

If it was pre-existing, should this patch not live at the start of this
series and carry a Cc: stable@kernel.org ?


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