[PATCH v7 tip/core/rcu 13/17] rcu: Make rcu_tasks_kthread()'s GP-wait loop allow preemption

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The grace-period-wait loop in rcu_tasks_kthread() is under (unnecessary)
RCU protection, and therefore has no preemption points in a PREEMPT=n
kernel.  This commit therefore removes the RCU protection and inserts
cond_resched().

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/update.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 2658de4a5975..f86d1ae50005 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -467,7 +467,7 @@ static void check_holdout_task(struct task_struct *t,
 	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
 	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
 		ACCESS_ONCE(t->rcu_tasks_holdout) = false;
-		list_del_rcu(&t->rcu_tasks_holdout_list);
+		list_del_init(&t->rcu_tasks_holdout_list);
 		put_task_struct(t);
 		return;
 	}
@@ -573,6 +573,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 			bool firstreport;
 			bool needreport;
 			int rtst;
+			struct task_struct *t1;
 
 			schedule_timeout_interruptible(HZ);
 			rtst = ACCESS_ONCE(rcu_task_stall_timeout);
@@ -582,11 +583,11 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 				lastreport = jiffies;
 			firstreport = true;
 			WARN_ON(signal_pending(current));
-			rcu_read_lock();
-			list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
-						rcu_tasks_holdout_list)
+			list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts,
+						rcu_tasks_holdout_list) {
 				check_holdout_task(t, needreport, &firstreport);
-			rcu_read_unlock();
+				cond_resched();
+			}
 		}
 
 		/*
-- 
1.8.1.5