On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paul...@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: sta...@kernel.org ? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
