[PATCH tip/core/rcu 14/20] rcu: Move rcu_report_unblock_qs_rnp() to common code

Paul E. McKenney
2015-03-03T17:52:19Z

The rcu_report_unblock_qs_rnp() function is invoked when the
last task blocking the current grace period exits its outermost
RCU read-side critical section. Previously, this was called only
from rcu_read_unlock_special(), and was therefore defined only when
CONFIG_RCU_PREEMPT=y. However, this function will be invoked even when
CONFIG_RCU_PREEMPT=n once CPU-hotplug operations are processed only at
the beginnings of RCU grace periods. The reason for this change is that
the last task on a given leaf rcu_node structure's ->blkd_tasks list
might well exit its RCU read-side critical section between the time that
recent CPU-hotplug operations were applied and when the new grace period
was initialized. This situation could result in RCU waiting forever on
that leaf rcu_node structure, because if all that structure's CPUs were
already offline, there would be no quiescent-state events to drive that
structure's part of the grace period.

This commit therefore moves rcu_report_unblock_qs_rnp() to common code
that is built unconditionally so that the quiescent-state-forcing code
can clean up after this situation, avoiding the grace-period stall.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/tree.c | 39 +++++++++++++++++++++++++++++++++++++++
kernel/rcu/tree_plugin.h | 40 ++--------------------------------------
2 files changed, 41 insertions(+), 38 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a7151d26b940..5b5cb1ff73ed 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2127,6 +2127,45 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
}

/*
+ * Record a quiescent state for all tasks that were previously queued
+ * on the specified rcu_node structure and that were blocking the current
+ * RCU grace period. The caller must hold the specified rnp->lock with
+ * irqs disabled, and this lock is released upon return, but irqs remain
+ * disabled.
+ */
+static void __maybe_unused rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
+ struct rcu_node *rnp, unsigned long flags)
+ __releases(rnp->lock)
+{
+ unsigned long mask;
+ struct rcu_node *rnp_p;
+
+ WARN_ON_ONCE(rsp == &rcu_bh_state || rsp == &rcu_sched_state);
+ if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ return; /* Still need more quiescent states! */
+ }
+
+ rnp_p = rnp->parent;
+ if (rnp_p == NULL) {
+ /*
+ * Either there is only one rcu_node in the tree,
+ * or tasks were kicked up to root rcu_node due to
+ * CPUs going offline.
+ */
+ rcu_report_qs_rsp(rsp, flags);
+ return;
+ }
+
+ /* Report up the rest of the hierarchy. */
+ mask = rnp->grpmask;
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ raw_spin_lock(&rnp_p->lock); /* irqs already disabled. */
+ smp_mb__after_unlock_lock();
+ rcu_report_qs_rnp(mask, rsp, rnp_p, flags);
+}
+
+/*
* Record a quiescent state for the specified CPU to that CPU's rcu_data
* structure. This must be either called from the specified CPU, or
* called when the specified CPU is known to be offline (and when it is
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ce8fb810770c..abccde8b893c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -232,43 +232,6 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
}

/*
- * Record a quiescent state for all tasks that were previously queued
- * on the specified rcu_node structure and that were blocking the current
- * RCU grace period. The caller must hold the specified rnp->lock with
- * irqs disabled, and this lock is released upon return, but irqs remain
- * disabled.
- */
-static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
- __releases(rnp->lock)
-{
- unsigned long mask;
- struct rcu_node *rnp_p;
-
- if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- return; /* Still need more quiescent states! */
- }
-
- rnp_p = rnp->parent;
- if (rnp_p == NULL) {
- /*
- * Either there is only one rcu_node in the tree,
- * or tasks were kicked up to root rcu_node due to
- * CPUs going offline.
- */
- rcu_report_qs_rsp(&rcu_preempt_state, flags);
- return;
- }
-
- /* Report up the rest of the hierarchy. */
- mask = rnp->grpmask;
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
- raw_spin_lock(&rnp_p->lock); /* irqs already disabled. */
- smp_mb__after_unlock_lock();
- rcu_report_qs_rnp(mask, &rcu_preempt_state, rnp_p, flags);
-}
-
-/*
* Advance a ->blkd_tasks-list pointer to the next entry, instead
* returning NULL if at the end of the list.
*/
@@ -398,7 +361,8 @@ void rcu_read_unlock_special(struct task_struct *t)
rnp->grplo,
rnp->grphi,
!!rnp->gp_tasks);
- rcu_report_unblock_qs_rnp(rnp, flags);
+ rcu_report_unblock_qs_rnp(&rcu_preempt_state,
+ rnp, flags);
} else {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
--
1.8.1.5