Message-ID: <4CDBE401.7040401@kernel.dk>
Date: Thu, 11 Nov 2010 13:39:29 +0100
From: Jens Axboe <axboe@kernel.dk>
MIME-Version: 1.0
To: Oleg Nesterov <oleg@redhat.com>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Greg Thelen <gthelen@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: INFO: suspicious rcu_dereference_check() usage -  kernel/pid.c:419
 invoked rcu_dereference_check() without protection!
References: <xr93fwwbdh1d.fsf@ninji.mtv.corp.google.com> <20101107182028.GZ15561@linux.vnet.ibm.com> <20101108151509.GA3702@redhat.com> <20101109202900.GV4032@linux.vnet.ibm.com> <20101110155530.GA1905@redhat.com> <20101110160211.GA2562@redhat.com> <4CDBD12C.4010807@kernel.dk> <20101111123015.GA25991@redhat.com>
In-Reply-To: <20101111123015.GA25991@redhat.com>
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On 2010-11-11 13:30, Oleg Nesterov wrote:
> 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().

Sure, not a big problem.

> 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.

Indeed...

> Hmm. And copy_io_context() has no callers ;)

Good find. It was previously used by the AS io scheduler, seems there
are no users left anymore. I queued up a patch to kill it.


-- 
Jens Axboe

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