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Message-ID: <202df44d-11e6-4dc6-8fbf-c89c0ceff73f@huaweicloud.com>
Date: Fri, 30 Jan 2026 14:07:48 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <llong@...hat.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Michal Koutný
<mkoutny@...e.com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
Anna-Maria Behnsen <anna-maria@...utronix.de>,
Frederic Weisbecker <frederic@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, Shuah Khan <shuah@...nel.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH/for-next 2/2] cgroup/cpuset: Introduce a new top level
isolcpus_update_mutex
On 2026/1/30 11:53, Waiman Long wrote:
> On 1/29/26 8:42 PM, Chen Ridong wrote:
>>
>> On 2026/1/30 9:35, Waiman Long wrote:
>>> On 1/29/26 7:56 PM, Chen Ridong wrote:
>>>> On 2026/1/30 5:16, Waiman Long wrote:
>>>>> On 1/29/26 3:01 AM, Chen Ridong wrote:
>>>>>> On 2026/1/28 12:42, Waiman Long wrote:
>>>>>>> The current cpuset partition code is able to dynamically update
>>>>>>> the sched domains of a running system and the corresponding
>>>>>>> HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the
>>>>>>> "isolcpus=domain,..." boot command line feature at run time.
>>>>>>>
>>>>>>> The housekeeping cpumask update requires flushing a number of different
>>>>>>> workqueues which may not be safe with cpus_read_lock() held as the
>>>>>>> workqueue flushing code may acquire cpus_read_lock() or acquiring locks
>>>>>>> which have locking dependency with cpus_read_lock() down the chain. Below
>>>>>>> is an example of such circular locking problem.
>>>>>>>
>>>>>>> ======================================================
>>>>>>> WARNING: possible circular locking dependency detected
>>>>>>> 6.18.0-test+ #2 Tainted: G S
>>>>>>> ------------------------------------------------------
>>>>>>> test_cpuset_prs/10971 is trying to acquire lock:
>>>>>>> ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at:
>>>>>>> touch_wq_lockdep_map+0x7a/0x180
>>>>>>>
>>>>>>> but task is already holding lock:
>>>>>>> ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at:
>>>>>>> cpuset_partition_write+0x85/0x130
>>>>>>>
>>>>>>> which lock already depends on the new lock.
>>>>>>>
>>>>>>> the existing dependency chain (in reverse order) is:
>>>>>>> -> #4 (cpuset_mutex){+.+.}-{4:4}:
>>>>>>> -> #3 (cpu_hotplug_lock){++++}-{0:0}:
>>>>>>> -> #2 (rtnl_mutex){+.+.}-{4:4}:
>>>>>>> -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}:
>>>>>>> -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}:
>>>>>>>
>>>>>>> Chain exists of:
>>>>>>> (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex
>>>>>>>
>>>>>>> 5 locks held by test_cpuset_prs/10971:
>>>>>>> #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at:
>>>>>>> ksys_write+0xf9/0x1d0
>>>>>>> #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at:
>>>>>>> kernfs_fop_write_iter+0x260/0x5f0
>>>>>>> #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at:
>>>>>>> kernfs_fop_write_iter+0x2b6/0x5f0
>>>>>>> #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at:
>>>>>>> cpuset_partition_write+0x77/0x130
>>>>>>> #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at:
>>>>>>> cpuset_partition_write+0x85/0x130
>>>>>>>
>>>>>>> Call Trace:
>>>>>>> <TASK>
>>>>>>> :
>>>>>>> touch_wq_lockdep_map+0x93/0x180
>>>>>>> __flush_workqueue+0x111/0x10b0
>>>>>>> housekeeping_update+0x12d/0x2d0
>>>>>>> update_parent_effective_cpumask+0x595/0x2440
>>>>>>> update_prstate+0x89d/0xce0
>>>>>>> cpuset_partition_write+0xc5/0x130
>>>>>>> cgroup_file_write+0x1a5/0x680
>>>>>>> kernfs_fop_write_iter+0x3df/0x5f0
>>>>>>> vfs_write+0x525/0xfd0
>>>>>>> ksys_write+0xf9/0x1d0
>>>>>>> do_syscall_64+0x95/0x520
>>>>>>> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>>>>>>
>>>>>>> To avoid such a circular locking dependency problem, we have to
>>>>>>> call housekeeping_update() without holding the cpus_read_lock()
>>>>>>> and cpuset_mutex. One way to do that is to introduce a new top level
>>>>>>> isolcpus_update_mutex which will be acquired first if the set of isolated
>>>>>>> CPUs may have to be updated. This new isolcpus_update_mutex will provide
>>>>>>> the need mutual exclusion without the need to hold cpus_read_lock().
>>>>>>>
>>>>>>> As cpus_read_lock() is now no longer held when
>>>>>>> tmigr_isolated_exclude_cpumask() is called, it needs to acquire it
>>>>>>> directly.
>>>>>>>
>>>>>>> The lockdep_is_cpuset_held() is also updated to check the new
>>>>>>> isolcpus_update_mutex.
>>>>>>>
>>>>>> I worry about the issue:
>>>>>>
>>>>>> CPU1 CPU2
>>>>>> rmdir
>>>>>> css->ss->css_killed(css);
>>>>>> cpuset_css_killed
>>>>>> __update_isolation_cpumasks
>>>>>> cpuset_full_unlock
>>>>>> css->flags |= CSS_DYING;
>>>>>> css_clear_dir(css);
>>>>>> ...
>>>>>> // offline and free do not
>>>>>> // get isolcpus_update_mutex
>>>>>> cpuset_css_offline
>>>>>> cpuset_css_free
>>>>>> cpuset_full_lock
>>>>>> ...
>>>>>> // UAF?
>>>>>>
>>>> Hi, Longman,
>>>>
>>>> In this patch, I noticed that cpuset_css_offline and cpuset_css_free do not
>>>> acquire the isolcpus_update_mutex. This could potentially lead to a UAF issue.
>>>>
>>>>> That is the reason why I add a new top-level isolcpus_update_mutex.
>>>>> cpuset_css_killed() and the update_isolation_cpumasks()'s unlock/lock sequence
>>>>> will have to acquire this isolcpus_update_mutex first.
>>>>>
>>>> However, simply adding isolcpus_update_mutex to cpuset_css_killed and
>>>> update_isolation_cpumasks may not be sufficient.
>>>>
>>>> As I mentioned, the path that calls __update_isolation_cpumasks may first
>>>> acquire isolcpus_update_mutex and cpuset_full_lock, but once cpuset_css_killed
>>>> is completed, it will release the “full” lock and then attempt to reacquire it
>>>> later. During this intermediate period, the cpuset may have already been freed,
>>>> because cpuset_css_offline and cpuset_css_free do not currently acquire the
>>>> isolcpus_update_mutex.
>>> You are right that acquisition of the new isolcpus_update_mutex should be in all
>>> the places where cpuset_full_lock() is acquired. Will update the patch to do
>>> that. That should eliminate the risk.
>>>
>> I suggest that putting isolcpus_update_mutex into cpuset_full_lock, since this
>> function means that all the locks needed have been acquired.
>>
>> void cpuset_full_lock(void)
>> {
>> mutex_lock(&isolcpus_update_mutex);
>> cpus_read_lock();
>> mutex_lock(&cpuset_mutex);
>> }
>>
>> void cpuset_full_unlock(void)
>> {
>> mutex_unlock(&cpuset_mutex);
>> cpus_read_unlock();
>> mutex_unlock(&isolcpus_update_mutex);
>> }
>
> That is what I had done.
>
Great.
--
Best regards,
Ridong
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