linux-kernel - Re: [-next v2 4/4] cpuset: add helpers for cpus read and cpuset

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Message-ID: <d09c4e49-8a3a-49b9-9f63-0b39a4bea45f@redhat.com>
Date: Wed, 13 Aug 2025 23:27:46 -0400
From: Waiman Long <llong@...hat.com>
To: Waiman Long <llong@...hat.com>, Chen Ridong <chenridong@...weicloud.com>,
 tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 lujialin4@...wei.com, chenridong@...wei.com, christophe.jaillet@...adoo.fr
Subject: Re: [-next v2 4/4] cpuset: add helpers for cpus read and cpuset_mutex
 locks

On 8/13/25 11:13 PM, Waiman Long wrote:
> On 8/13/25 8:44 PM, Chen Ridong wrote:
>>
>> On 2025/8/14 4:09, Waiman Long wrote:
>>> On 8/13/25 4:29 AM, Chen Ridong wrote:
>>>> From: Chen Ridong <chenridong@...wei.com>
>>>>
>>>> cpuset: add helpers for cpus_read_lock and cpuset_mutex
>>>>
>>>> Replace repetitive locking patterns with new helpers:
>>>> - cpus_read_cpuset_lock()
>>>> - cpus_read_cpuset_unlock()
>>>>
>>>> This makes the code cleaner and ensures consistent lock ordering.
>>>>
>>>> Signed-off-by: Chen Ridong <chenridong@...wei.com>
>>>> ---
>>>>    kernel/cgroup/cpuset-internal.h |  2 ++
>>>>    kernel/cgroup/cpuset-v1.c       | 12 +++------
>>>>    kernel/cgroup/cpuset.c          | 48 
>>>> +++++++++++++++------------------
>>>>    3 files changed, 28 insertions(+), 34 deletions(-)
>>>>
>>>> diff --git a/kernel/cgroup/cpuset-internal.h 
>>>> b/kernel/cgroup/cpuset-internal.h
>>>> index 75b3aef39231..6fb00c96044d 100644
>>>> --- a/kernel/cgroup/cpuset-internal.h
>>>> +++ b/kernel/cgroup/cpuset-internal.h
>>>> @@ -276,6 +276,8 @@ int cpuset_update_flag(cpuset_flagbits_t bit, 
>>>> struct cpuset *cs, int turning_on)
>>>>    ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>>>>                        char *buf, size_t nbytes, loff_t off);
>>>>    int cpuset_common_seq_show(struct seq_file *sf, void *v);
>>>> +void cpus_read_cpuset_lock(void);
>>>> +void cpus_read_cpuset_unlock(void);
>>> The names are not intuitive. I would prefer just extend the 
>>> cpuset_lock/unlock to include
>>> cpus_read_lock/unlock and we use cpuset_lock/unlock consistently in 
>>> the cpuset code. Also, there is
>>> now no external user of cpuset_lock/unlock, we may as well remove 
>>> them from include/linux/cpuset.h.
>>>
>>> Cheers,
>>> Longman
>> I like the idea and have considered it.
>> However, I noticed that cpuset_locked is being used in 
>> __sched_setscheduler.
>
> Right, I overloooked the cpuset_lock() call in kernel/sched/syscall.c. 
> So we can't remove it from include/linux/cpuset.h.
>
> This call is invoked to ensure cpusets information is stable. However, 
> it doesn't hurt if the cpus_read_lock() is also acquired as a result. 
> Alternatively, we can use a name like cpuset_full_lock() to include 
> cpus_read_lock().

I have a correction. According to commit d74b27d63a8b ("cgroup/cpuset: 
Change cpuset_rwsem and hotplug lock order") , sched_scheduler() can be 
called while holding cpus_hotplug_lock. So we should keep cpuset_lock() 
as it is.

Cheers,
Longman