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 13:30:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
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 11/11, Jens Axboe wrote:
>
> On 2010-11-10 17:02, Oleg Nesterov wrote:
> >
> > 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.

Probably. After that (perhaps) there was another reason, see

	5b160f5e "copy_process: cosmetic ->ioprio tweak"
	cf342e52 "Don't need to disable interrupts for tasklist_lock"

But this was dismissed by

	fd0928df "ioprio: move io priority from task_struct to io_context"

> 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().

Yes, I thought about this too. The only drawback is that we should
take task_lock() unconditionally in exit_io_context().

Btw, in theory get_task_ioprio() is racy too. "ret = p->io_context->ioprio"
can lead to use-after-free. Probably needs task_lock() as well.

Hmm. And copy_io_context() has no callers ;)

Oleg.

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