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Wed, 20 Jul 2022 03:27:43 +0000 Received: from CO1PR10MB4468.namprd10.prod.outlook.com ([fe80::dd9d:e9dd:d864:524c]) by CO1PR10MB4468.namprd10.prod.outlook.com ([fe80::dd9d:e9dd:d864:524c%9]) with mapi id 15.20.5458.018; Wed, 20 Jul 2022 03:27:43 +0000 Message-ID: Date: Wed, 20 Jul 2022 13:27:28 +1000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Query regarding deadlock involving cgroup_threadgroup_rwsem and cpu_hotplug_lock Content-Language: en-US From: Imran Khan To: tj@kernel.org, lizefan.x@bytedance.com, hannes@cmpxchg.org, tglx@linutronix.de, steven.price@arm.com, peterz@infradead.org Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org References: <8245b710-8acb-d8e6-7045-99a5f71dad4e@oracle.com> <26d0e4cc-be0e-2c12-6174-dfbb1edb1ed6@oracle.com> In-Reply-To: <26d0e4cc-be0e-2c12-6174-dfbb1edb1ed6@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SG2PR04CA0185.apcprd04.prod.outlook.com (2603:1096:4:14::23) To CO1PR10MB4468.namprd10.prod.outlook.com (2603:10b6:303:6c::24) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 4e2b4c00-8595-4fe7-39bf-08da69ffce40 X-MS-TrafficTypeDiagnostic: DM6PR10MB2921:EE_ X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Due to some missing drivers I don't have this test setup for latest upstream kernel but looking at the code the issue seems to be present in the latest kernel as well. If needed I can provide stack traces and other relevant info from the vmcore that I have got from 5.4 setup. The description of the problem is as follows (I am using 5.19-rc7 as reference below): __cgroup_procs_write acquires cgroup_threadgroup_rwsem via cgroup_procs_write_start and then invokes cgroup_attach_task. Now cgroup_attach_task can invoke following call chain: cgroup_attach_task --> cgroup_migrate --> cgroup_migrate_execute --> cpuset_attach Here cpuset_attach tries to take cpu_hotplug_lock. But by this time if some other context 1. is already in the middle of cpu hotplug and has acquired cpu_hotplug_lock in _cpu_up but 2. has not yet reached CPUHP_ONLINE state and 3. one of the intermediate hotplug states (in my case CPUHP_AP_ONLINE_DYN ) has a callback which involves creation of a thread (or invocation of copy_process via some other path) the invoked copy_process will get blocked on cgroup_threadgroup_rwsem in following call chain: copy_process --> cgroup_can_fork --> cgroup_css_set_fork --> cgroup_threadgroup_change_begin I am looking for suggestions to fix this deadlock. Or if I am missing something in the above analysis and the above mention scenario can't happen in latest upstream kernel, then please let me know as that would help me in back porting relevant changes to 5.4 kernel because the issue definitely exists in 5.4 kernel. Thanks, -- Imran