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

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Message-ID: <93e37ccf-8ac8-40f5-840f-2f221f58131e@redhat.com>
Date: Wed, 13 Aug 2025 23:13:44 -0400
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Waiman Long <llong@...hat.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 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().

Cheers,
Longman