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Consider the following sequence of events in a PREEMPT=y kernel: 1. All but one of the CPUs corresponding to a given leaf rcu_node structure go offline. Each of these CPUs clears its bit in that structure's ->qsmaskinitnext field. 2. A new grace period starts, and rcu_gp_init() scans the leaf rcu_node structures, applying CPU-hotplug changes since the start of the previous grace period, including those changes in #1 above. This copies each leaf structure's ->qsmaskinitnext to its ->qsmask field, which represents the CPUs that this new grace period will wait on. Each copy operation is done holding the corresponding leaf rcu_node structure's ->lock, and at the end of this scan, rcu_gp_init() holds no locks. 3. The last CPU corresponding to #1's leaf rcu_node structure goes offline, clearing its bit in that structure's ->qsmaskinitnext field, but not touching the ->qsmaskinit field. Note that rcu_gp_init() is not currently holding any locks! This CPU does -not- report a quiescent state because the grace period has not yet initialized itself sufficiently to have set any bits in any of the leaf rcu_node structures' ->qsmask fields. 4. The rcu_gp_init() function continues initializing the new grace period, copying each leaf rcu_node structure's ->qsmaskinit field to its ->qsmask field while holding the corresponding ->lock. This sets the ->qsmask bit corresponding to #3's CPU. 5. Before the grace period ends, #3's CPU comes back online. Because te grace period has not yet done any force-quiescent-state scans (which would report a quiescent state on behalf of any offline CPUs), this CPU's ->qsmask bit is still set. 6. A task running on the newly onlined CPU is preempted while in an RCU read-side critical section. Because this CPU's ->qsmask bit is net, not only does this task queue itself on the leaf rcu_node structure's ->blkd_tasks list, it also sets that structure's ->gp_tasks pointer to reference it. 7. The grace period started in #1 above comes to an end. This results in rcu_gp_cleanup() being invoked, which, among other things, checks to make sure that there are no tasks blocking the just-ended grace period, that is, that all ->gp_tasks pointers are NULL. The ->gp_tasks pointer corresponding to the task preempted in #3 above is non-NULL, which results in a splat. This splat is a false positive. The task's RCU read-side critical section cannot have begun before the just-ended grace period because this would mean either: (1) The CPU came online before the grace period started, which cannot have happened because the grace period started before that CPU went offline, or (2) The task started its RCU read-side critical section on some other CPU, but then it would have had to have been preempted before migrating to this CPU, which would mean that it would have instead queued itself on that other CPU's rcu_node structure. RCU's grace periods thus are working correctly. Or, more accurately, that remaining bugs in RCU's grace periods are elsewhere. This commit eliminates this false positive by adding code to the end of rcu_cpu_starting() that reports a quiescent state to RCU, which has the side-effect of clearing that CPU's ->qsmask bit, preventing the above scenario. This approach has the added benefit of more promptly reporting quiescent states corresponding to offline CPUs. Nevertheless, this commit does -not- remove the need for the force-quiescent-state scans to check for offline CPUs, given that a CPU might remain offline indefinitely. And without the checks in the force-quiescent-state scans, the grace period would also persist indefinitely, which could result in hangs or memory exhaustion. Note well that the call to rcu_report_qs_rnp() reporting the quiescent state must come -after- the setting of this CPU's bit in the leaf rcu_node structure's ->qsmaskinitnext field. Otherwise, lockdep-RCU will complain bitterly about quiescent states coming from an offline CPU. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> |
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| .. | ||
| Kconfig | ||
| Kconfig.debug | ||
| Makefile | ||
| rcu_segcblist.c | ||
| rcu_segcblist.h | ||
| rcu.h | ||
| rcuperf.c | ||
| rcutorture.c | ||
| srcutiny.c | ||
| srcutree.c | ||
| sync.c | ||
| tiny.c | ||
| tree_exp.h | ||
| tree_plugin.h | ||
| tree.c | ||
| tree.h | ||
| update.c | ||