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Subject[PATCH 1/1] 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

- 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 <>
Acked-by: Frederic Weisbecker <>
Acked-by: Mandeep Singh Baines <>
Acked-by: Paul E. McKenney <>
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;
+ can_cont = pid_alive(g) && pid_alive(t);
+ 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 */

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