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rcu: Yet another fix for preemption and CPU hotplug
As noted earlier, the following sequence of events can occur when running PREEMPT_RCU and HOTPLUG_CPU on a system with a multi-level rcu_node combining tree: 1. A group of tasks block on CPUs corresponding to a given leaf rcu_node structure while within RCU read-side critical sections. 2. All CPUs corrsponding to that rcu_node structure go offline. 3. The next grace period starts, but because there are still tasks blocked, the upper-level bits corresponding to this leaf rcu_node structure remain set. 4. All the tasks exit their RCU read-side critical sections and remove themselves from the leaf rcu_node structure's list, leaving it empty. 5. But because there now is code to check for this condition at force-quiescent-state time, the upper bits are cleared and the grace period completes. However, there is another complication that can occur following step 4 above: 4a. The grace period starts, and the leaf rcu_node structure's gp_tasks pointer is set to NULL because there are no tasks blocked on this structure. 4b. One of the CPUs corresponding to the leaf rcu_node structure comes back online. 4b. An endless stream of tasks are preempted within RCU read-side critical sections on this CPU, such that the ->blkd_tasks list is always non-empty. The grace period will never end. This commit therefore makes the force-quiescent-state processing check only for absence of tasks blocking the current grace period rather than absence of tasks altogether. This will cause a quiescent state to be reported if the current leaf rcu_node structure is not blocking the current grace period and its parent thinks that it is, regardless of how RCU managed to get itself into this state. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: <stable@vger.kernel.org> # 4.0.x Tested-by: Sasha Levin <sasha.levin@oracle.com>
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commit
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1 changed files with 27 additions and 16 deletions
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@ -2199,8 +2199,8 @@ static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
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unsigned long mask;
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struct rcu_node *rnp_p;
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WARN_ON_ONCE(rsp == &rcu_bh_state || rsp == &rcu_sched_state);
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if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
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if (rcu_state_p == &rcu_sched_state || rsp != rcu_state_p ||
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rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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return; /* Still need more quiescent states! */
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}
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@ -2208,9 +2208,8 @@ static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
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rnp_p = rnp->parent;
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if (rnp_p == NULL) {
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/*
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* Either there is only one rcu_node in the tree,
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* or tasks were kicked up to root rcu_node due to
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* CPUs going offline.
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* Only one rcu_node structure in the tree, so don't
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* try to report up to its nonexistent parent!
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*/
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rcu_report_qs_rsp(rsp, flags);
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return;
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@ -2713,8 +2712,29 @@ static void force_qs_rnp(struct rcu_state *rsp,
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return;
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}
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if (rnp->qsmask == 0) {
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rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
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continue;
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if (rcu_state_p == &rcu_sched_state ||
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rsp != rcu_state_p ||
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rcu_preempt_blocked_readers_cgp(rnp)) {
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/*
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* No point in scanning bits because they
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* are all zero. But we might need to
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* priority-boost blocked readers.
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*/
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rcu_initiate_boost(rnp, flags);
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/* rcu_initiate_boost() releases rnp->lock */
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continue;
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}
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if (rnp->parent &&
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(rnp->parent->qsmask & rnp->grpmask)) {
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/*
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* Race between grace-period
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* initialization and task exiting RCU
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* read-side critical section: Report.
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*/
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rcu_report_unblock_qs_rnp(rsp, rnp, flags);
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/* rcu_report_unblock_qs_rnp() rlses ->lock */
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continue;
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}
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}
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cpu = rnp->grplo;
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bit = 1;
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@ -2729,15 +2749,6 @@ static void force_qs_rnp(struct rcu_state *rsp,
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if (mask != 0) {
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/* Idle/offline CPUs, report. */
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rcu_report_qs_rnp(mask, rsp, rnp, flags);
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} else if (rnp->parent &&
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list_empty(&rnp->blkd_tasks) &&
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!rnp->qsmask &&
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(rnp->parent->qsmask & rnp->grpmask)) {
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/*
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* Race between grace-period initialization and task
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* existing RCU read-side critical section, report.
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*/
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rcu_report_unblock_qs_rnp(rsp, rnp, flags);
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} else {
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/* Nothing to do here, so just drop the lock. */
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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