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

From: Mandeep Singh Baines <msb@google.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Don Zickus <dzickus@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@elte.hu>,
	Jerome Marchand <jmarchan@redhat.com>,
	Roland McGrath <roland@redhat.com>,
	linux-kernel@vger.kernel.org, stable@kernel.org
Subject: Re: [PATCH] fix the racy check_hung_uninterruptible_tasks()->rcu_lock_break() logic
Date: Fri, 18 Jun 2010 13:38:54 -0700	[thread overview]
Message-ID: <20100618203854.GW915@google.com> (raw)
In-Reply-To: <20100618190251.GA17297@redhat.com>

Oleg Nesterov (oleg@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@redhat.com>
> ---

Acked-by: Mandeep Singh Baines <msb@google.com>