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From: Oleg Nesterov on 18 Jun 2010 15:10
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.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com>
---

kernel/hung_task.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

--- 35-rc2/kernel/hung_task.c~CHT_FIX_RCU_LOCK_BREAK 2009-12-18 19:05:38.000000000 +0100
+++ 35-rc2/kernel/hung_task.c 2010-06-18 20:06:11.000000000 +0200
@@ -113,15 +113,20 @@ static void check_hung_task(struct task_
* For preemptible RCU it is sufficient to call rcu_read_unlock in order
* 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;
}

/*
@@ -148,9 +153,7 @@ static void check_hung_uninterruptible_t
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 */

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