linux-kernel - Re: [PATCH 3/3] cgroup/cpuset: Allow only one active attach operation per cpuset

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Date:   Mon, 3 Apr 2023 13:41:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Christian Brauner <brauner@...nel.org>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        gscrivan@...hat.com
Subject: Re: [PATCH 3/3] cgroup/cpuset: Allow only one active attach operation
 per cpuset


On 4/3/23 12:47, Michal Koutný wrote:
> On Fri, Mar 31, 2023 at 10:50:45AM -0400, Waiman Long <longman@...hat.com> wrote:
>> The current cpuset code uses the global cpuset_attach_old_cs variable
>> to store the old cpuset value between consecutive cpuset_can_attach()
>> and cpuset_attach() calls. Since a caller of cpuset_can_attach() may
>> not need to hold the global cgroup_threadgroup_rwsem, parallel cpuset
>> attach operations are possible.
> Do I understand correctly this consequence of the cpuset_attach_task()
> on the clone path?
> In that particular case (with CLONE_INTO_CGROUP) cgroup_mutex is taken,
> so the access the the old_cs variable should still be synchronized with
> regular migrations that are also under cgroup_mutex.

This patch is actually not related to the CLONE_INTO_GROUP problem in 
patch 1. It is a generic problem when multiple users are moving threads 
into cgroup.threads of the same or different cpusets simultaneously.


>> When there are concurrent cpuset attach operations in progress,
>> cpuset_attach() may fetch the wrong value from cpuset_attach_old_cs
>> causing incorrect result.  To avoid this problem while still allowing
>> certain level of parallelism, drop cpuset_attach_old_cs and use a
>> per-cpuset attach_old_cs value. Also restrict to at most one active
>> attach operation per cpuset to avoid corrupting the value of the
>> per-cpuset attach_old_cs value.
> Secondly, semantically wouldn't a `void *ss_priv[CGROUP_SUBSYS_COUNT]`
> in struct cgroup_taskset make it simpler wrt the exclusivity guarantees?

I guess we can put the old_cs value into cgroup_taskset. Since the 
related attach_in_progress value is in cpuset, so I put the old_cs there 
too.


> Thirdly, if my initial assumptino is right -- I'd suggest ordering this
> before the patch `cgroup/cpuset: Make cpuset_fork() handle
> CLONE_INTO_CGROUP properly` to spare backporters possible troubles if
> this is would be a fixup to that.

I don't believe this patch has a dependency on patch 1.

I had thought about adding a fixes tag so it will be backported to 
stable. However, this problem should be rare. Let's see what others 
think about it.

Cheers,
Longman