linux-kernel - Re: [PATCH] fix the racy check_hung_uninterruptible_tasks()->rcu_lock

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Date:	Fri, 18 Jun 2010 22:11:52 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Jerome Marchand <jmarchan@...hat.com>,
	Mandeep Singh Baines <msb@...gle.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH] fix the racy
	check_hung_uninterruptible_tasks()->rcu_lock_break() logic

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@...hat.com>



Very nice!

Acked-by: Frederic Weisbecker <fweisbec@...il.com>

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