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Commit 6027ce49 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

hung_task: fix the broken rcu_lock_break() logic 



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: default avatarOleg Nesterov <oleg@redhat.com>
Acked-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
Acked-by: default avatarMandeep Singh Baines <msb@google.com>
Acked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6e27f63e

kernel/hung_task.c

+7 −4
Original line number Diff line number Diff line
@@ -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 */