rcu: Make TASKS_RCU handle nohz_full= CPUs

Currently TASKS_RCU would ignore a CPU running a task in nohz_full=
usermode execution.  There would be neither a context switch nor a
scheduling-clock interrupt to tell TASKS_RCU that the task in question
had passed through a quiescent state.  The grace period would therefore
extend indefinitely.  This commit therefore makes RCU's dyntick-idle
subsystem record the task_struct structure of the task that is running
in dyntick-idle mode on each CPU.  The TASKS_RCU grace period can
then access this information and record a quiescent state on
behalf of any CPU running in dyntick-idle usermode.

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

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index e1d71741958f..2658de4a5975 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -463,7 +463,9 @@ static void check_holdout_task(struct task_struct *t,
 {
         if (!ACCESS_ONCE(t->rcu_tasks_holdout) ||
             t->rcu_tasks_nvcsw != ACCESS_ONCE(t->nvcsw) ||
-            !ACCESS_ONCE(t->on_rq)) {
+            !ACCESS_ONCE(t->on_rq) ||
+            (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);
                 put_task_struct(t);