rcu: Make rcu_read_unlock_special() propagate ->qsmaskinit bit clearing

This commit causes rcu_read_unlock_special() to propagate ->qsmaskinit
bit clearing up the rcu_node tree once a given rcu_node structure's
blkd_tasks list becomes empty.  This is the final commit in preparation
for the rework of RCU priority boosting:  It enables preempted tasks to
remain queued on their rcu_node structure even after all of that rcu_node
structure's CPUs have gone offline.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

2 files changed, 17 insertions, 3 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 75c6b3301abb..6625a1b5d9a1 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2329,6 +2329,10 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
 {
 }
 
+static void __maybe_unused rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
+{
+}
+
 static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
 {
 }
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d044b9cbbd97..8a2b84157d34 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -322,9 +322,10 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
  */
 void rcu_read_unlock_special(struct task_struct *t)
 {
-        int empty_exp;
-        int empty_norm;
-        int empty_exp_now;
+        bool empty;
+        bool empty_exp;
+        bool empty_norm;
+        bool empty_exp_now;
         unsigned long flags;
         struct list_head *np;
 #ifdef CONFIG_RCU_BOOST
@@ -376,6 +377,7 @@ void rcu_read_unlock_special(struct task_struct *t)
                                 break;
                         raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
                 }
+                empty = !rcu_preempt_has_tasks(rnp);
                 empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
                 empty_exp = !rcu_preempted_readers_exp(rnp);
                 smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
@@ -396,6 +398,14 @@ void rcu_read_unlock_special(struct task_struct *t)
 #endif /* #ifdef CONFIG_RCU_BOOST */
 
                 /*
+                 * If this was the last task on the list, go see if we
+                 * need to propagate ->qsmaskinit bit clearing up the
+                 * rcu_node tree.
+                 */
+                if (!empty && !rcu_preempt_has_tasks(rnp))
+                        rcu_cleanup_dead_rnp(rnp);
+
+                /*
                  * If this was the last task on the current list, and if
                  * we aren't waiting on any CPUs, report the quiescent state.
                  * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,