linux-kernel - Re: [cgroup/for-6.19 PATCH] cgroup/cpuset: Make callback_lock a raw_spinlock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <da750417-948c-4968-bdb3-9fd267fb9c10@redhat.com>
Date: Thu, 13 Nov 2025 11:35:41 -0500
From: Waiman Long <llong@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Waiman Long <llong@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev, Chen Ridong <chenridong@...wei.com>,
 Pingfan Liu <piliu@...hat.com>, Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [cgroup/for-6.19 PATCH] cgroup/cpuset: Make callback_lock a
 raw_spinlock_t

On 11/13/25 2:53 AM, Sebastian Andrzej Siewior wrote:
> On 2025-11-12 13:21:12 [-0500], Waiman Long wrote:
>> On 11/12/25 3:51 AM, Sebastian Andrzej Siewior wrote:
>>> On 2025-11-11 22:57:59 [-0500], Waiman Long wrote:
>>>> The callback_lock is a spinlock_t which is acquired either to read
>>>> a stable set of cpu or node masks or to modify those masks when
>>>> cpuset_mutex is also acquired. Sometime it may need to go up the
>>>> cgroup hierarchy while holding the lock to find the right set of masks
>>>> to use. Assuming that the depth of the cgroup hierarch is finite and
>>>> typically small, the lock hold time should be limited.
>>> We can't assume that, can we?
>> We can theoretically create a cgroup hierarchy with many levels, but no sane
>> users will actually do that. If this is a concern to you, I can certainly
>> drop this patch.
> Someone will think this is sane and will wonder. We usually don't impose
> limits but make sure things are preemptible so it does not matter.
>
>>>> Some externally callable cpuset APIs like cpuset_cpus_allowed() and
>>> cpuset_cpus_allowed() has three callers in kernel/sched/ and all use
>>> GFP_KERNEL shortly before invoking the function in question.
>> The current callers of these APIs are fine. What I am talking is about new
>> callers that may want to call them when holding a raw_spinlock_t.
> No, please don't proactive do these changes like this which are not
> fixes because something was/ is broken.

I posted this patch as a response to my review of Pingfan Liu's deadline 
scheduler patch as it need to call cpuset_cpus_allowed() to get a proper 
cpumask. However, there is an alternative way to solve this problem, so 
I am dropping this patch now. In the future, if there is a better use 
case that needs this change, I will push this patch again.

Thanks,
Longman