linux-kernel - Re: [PATCH v3 1/2] x86/resctrl: IPI all CPUs for group updates

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Date:   Wed, 23 Nov 2022 08:45:41 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Peter Newman <peternewman@...gle.com>
CC:     <fenghua.yu@...el.com>, <bp@...en8.de>, <derkling@...gle.com>,
        <eranian@...gle.com>, <hpa@...or.com>, <james.morse@....com>,
        <jannh@...gle.com>, <kpsingh@...gle.com>,
        <linux-kernel@...r.kernel.org>, <mingo@...hat.com>,
        <tglx@...utronix.de>, <x86@...nel.org>
Subject: Re: [PATCH v3 1/2] x86/resctrl: IPI all CPUs for group updates

Hi Peter,

On 11/23/2022 3:09 AM, Peter Newman wrote:
> On Mon, Nov 21, 2022 at 10:53 PM Reinette Chatre
> <reinette.chatre@...el.com> wrote:
>> On 11/15/2022 6:19 AM, Peter Newman wrote:

...

>>> search the entire tasklist for group members which could be running on
>>> that CPU. This needs to be done while blocking updates to the tasklist
>>> to avoid leaving newly-created child tasks assigned to the old
>>> CLOSID/RMID.
>>
>> This is not clear to me. rdt_move_group_tasks() obtains a read lock,
>> read_lock(&tasklist_lock), so concurrent modifications to the tasklist
>> are indeed possible. Should this perhaps be write_lock() instead?
>> It sounds like the scenario you are describing may be a concern. That is,
>> if a task belonging to a group that is being removed happens to
>> call fork()/clone() during the move then the child may end up being
>> created with old closid.
> 
> Shouldn't read_lock(&tasklist_lock) cause write_lock(&tasklist_lock) to
> block?

Apologies, yes, you are right. I was focusing too much on the detail of
task_struct::tasks being a RCU list and lost sight of the actual lock
obtained (rcu_read_lock() vs read_lock()) in this instance. 

Reinette