Re: [PATCH] fix the racycheck_hung_uninterruptible_tasks()->rcu_lock_break() logic

[Mandeep Singh Baines]

Oleg Nesterov (oleg@xxxxxxxxxx) 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@xxxxxxxxxx>
> ---

Acked-by: Mandeep Singh Baines <msb@xxxxxxxxxx>
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