linux-kernel - Re: INFO: suspicious rcu_dereference_check() usage - kernel/pid.c:419 invoked rcu_dereference

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Date:	Thu, 11 Nov 2010 12:19:08 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: INFO: suspicious rcu_dereference_check() usage -  kernel/pid.c:419
 invoked rcu_dereference_check() without protection!

On 2010-11-10 17:02, Oleg Nesterov wrote:
> (another try with the proper email address)
> 
> On 11/09, Paul E. McKenney wrote:
>>
>> Thank you, Oleg!  Greg, would you be willing to update your patch
>> to remove the comment?  (Perhaps tasklist_lock as well...)
> 
> Agreed, I think tasklock should be killed.
> 
> 
> But wait. Whatever we do, isn't this code racy? I do not see why, say,
> sys_ioprio_set(IOPRIO_WHO_PROCESS) can't install ->io_context after
> this task has already passed exit_io_context().
> 
> Jens, am I missed something?

Not sure, I think the original intent was for the tasklist_lock to
protect from a concurrent exit, but that looks like nonsense and it was
just there to protect the task lookup.

How about moving the ->io_context check and exit_io_context() in
do_exit() under the task lock? Coupled with a check for PF_EXITING in
set_task_ioprio().


-- 
Jens Axboe

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