[PATCH 2/2] hung_task: improve the rcu_lock_break() logic

[Oleg Nesterov <oleg@redhat.com>]

check_hung_uninterruptible_tasks() stops after rcu_lock_break() if either
"t" or "g" exits, this is suboptimal.

If "t" is alive, we can always continue, t->group_leader can be used as the
new "g". We do not even bother to check g != NULL in this case.

If "g" is alive, we can at least continue the outer for_each_process() loop.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/hung_task.c |   29 ++++++++++++++++++++---------
 1 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 4735b99..f488059 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -134,20 +134,26 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
  * For preemptible RCU it is sufficient to call rcu_read_unlock in order
  * to exit the grace period. For classic RCU, a reschedule is required.
  */
-static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
+static void rcu_lock_break(struct task_struct **g, struct task_struct **t)
 {
-	bool can_cont;
+	bool alive;
+
+	get_task_struct(*g);
+	get_task_struct(*t);
 
-	get_task_struct(g);
-	get_task_struct(t);
 	rcu_read_unlock();
 	cond_resched();
 	rcu_read_lock();
-	can_cont = pid_alive(g) && pid_alive(t);
-	put_task_struct(t);
-	put_task_struct(g);
 
-	return can_cont;
+	alive = pid_alive(*g);
+	put_task_struct(*g);
+	if (!alive)
+		*g = NULL;
+
+	alive = pid_alive(*t);
+	put_task_struct(*t);
+	if (!alive)
+		*t = NULL;
 }
 
 /*
@@ -178,7 +184,12 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 
 			if (!--batch_count) {
 				batch_count = HUNG_TASK_BATCHING;
-				if (!rcu_lock_break(g, t))
+				rcu_lock_break(&g, &t);
+				if (t)		/* in case g == NULL */
+					g = t->group_leader;
+				else if (g)	/* continue the outer loop */
+					break;
+				else		/* both dead */
 					goto unlock;
 			}
 			/* use "==" to skip the TASK_KILLABLE tasks */
-- 
1.5.5.1