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: 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>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 @@
* 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 @@
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 */
