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([2601:188:c102:b180:1f8b:71d0:77b1:1f6e]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-894d36ad309sm49846046d6.6.2026.01.29.17.35.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Jan 2026 17:35:27 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <80842353-e054-4c70-a560-f67401c5b4a2@redhat.com> Date: Thu, 29 Jan 2026 20:35:26 -0500 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH/for-next 2/2] cgroup/cpuset: Introduce a new top level isolcpus_update_mutex To: Chen Ridong , Waiman Long , Tejun Heo , Johannes Weiner , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Anna-Maria Behnsen , Frederic Weisbecker , Thomas Gleixner , Shuah Khan Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260128044251.1229702-1-longman@redhat.com> <20260128044251.1229702-3-longman@redhat.com> <08c3fad6-b881-4089-b081-bde6efbafbd2@huaweicloud.com> <11d8ff97-74e5-440e-b56a-af590da5a3f6@huaweicloud.com> Content-Language: en-US In-Reply-To: <11d8ff97-74e5-440e-b56a-af590da5a3f6@huaweicloud.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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: >>>>     >>>>       : >>>>     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. Cheers, Longman