rcu: Make rcu_tasks_kthread()'s GP-wait loop allow preemption

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>

1 files 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();
+                        }
                 }
 
                 /*