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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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