[PATCH 1/1] hung_task: fix the broken rcu_lock_break() logic

[Oleg Nesterov <oleg@redhat.com>]

check_hung_uninterruptible_tasks()->rcu_lock_break() introduced by
"softlockup: check all tasks in hung_task" commit ce9dbe24 looks
absolutely wrong.

	- rcu_lock_break() does put_task_struct(). If the task has exited
	  it is not safe to even read its ->state, nothing protects this
	  task_struct.

	- The TASK_DEAD checks are wrong too. Contrary to the comment, we
	  can't use it to check if the task was unhashed. It can be unhashed
	  without TASK_DEAD, or it can be valid with TASK_DEAD.

	  For example, an autoreaping task can do release_task(current)
	  long before it sets TASK_DEAD in do_exit().

	  Or, a zombie task can have ->state == TASK_DEAD but release_task()
	  was not called, and in this case we must not break the loop.

Change this code to check pid_alive() instead, and do this before
we drop the reference to the task_struct.

Note: while_each_thread() under rcu_read_lock() is not really safe,
it can livelock. This will be fixed later, but fortunately in this
case the "max_count" logic saves us anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Mandeep Singh Baines <msb@google.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/hung_task.c |   11 +++++++----
 1 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 2e48ec0..c21449f 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -119,15 +119,20 @@ 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 void rcu_lock_break(struct task_struct *g, struct task_struct *t)
+static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
 {
+	bool can_cont;
+
 	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;
 }
 
 /*
@@ -154,9 +159,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 			goto unlock;
 		if (!--batch_count) {
 			batch_count = HUNG_TASK_BATCHING;
-			rcu_lock_break(g, t);
-			/* Exit if t or g was unhashed during refresh. */
-			if (t->state == TASK_DEAD || g->state == TASK_DEAD)
+			if (!rcu_lock_break(g, t))
 				goto unlock;
 		}
 		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
-- 
1.5.5.1