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From: Frederic Weisbecker on 18 Jun 2010 16:20
On Fri, Jun 18, 2010 at 09:02:51PM +0200, Oleg Nesterov wrote:
> 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>



Very nice!

Acked-by: Frederic Weisbecker <fweisbec(a)gmail.com>

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From: Mandeep Singh Baines on 18 Jun 2010 16:50
Oleg Nesterov (oleg(a)redhat.com) wrote:
> 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>
> ---

Acked-by: Mandeep Singh Baines <msb(a)google.com>
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