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The current approach to grace-period initialization is vulnerable to extremely low-probability races. These races stem from 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. This means that some rcu_node structures will believe that the old grace period is still in effect at the same time that other rcu_node structures believe that the new grace period has already started. These sorts of disagreements 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 structures 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 CPU believes that its 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 -- which has not yet been made aware of the new grace period. 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 because it corresponds to the second as-yet-uninitialized rcu_node structure, 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_DONE_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. That said, it would have been insanely difficult to force this race to happen before the grace-period initialization process was preemptible. Therefore, this commit is not a candidate for -stable. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Conflicts: kernel/rcutree.c |
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