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Mon, 8 Jul 2024 17:48:00 +0000 Message-ID: <20735ea0-2a3a-4230-92c6-6007c0777e24@intel.com> Date: Mon, 8 Jul 2024 10:47:57 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 31/38] x86/resctrl: resctrl_exit() teardown resctrl but leave the mount point To: James Morse , , CC: Fenghua Yu , Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Babu Moger , , D Scott Phillips OS , , , , , , Jamie Iles , Xin Hao , , , , David Hildenbrand , Rex Nie , "Dave Martin" , Shaopeng Tan References: <20240614150033.10454-1-james.morse@arm.com> <20240614150033.10454-32-james.morse@arm.com> <4b988afb-bc2e-4b4c-8ebe-e1db0b614f24@arm.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <4b988afb-bc2e-4b4c-8ebe-e1db0b614f24@arm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW2PR16CA0021.namprd16.prod.outlook.com (2603:10b6:907::34) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|IA1PR11MB6145:EE_ X-MS-Office365-Filtering-Correlation-Id: 327c1e61-3a49-4fe5-6309-08dc9f761c06 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|1800799024|376014|366016; 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MPAM has an error interrupt which indicates >>> the MPAM driver has gone haywire. Should this occur tasks could run with >>> the wrong control values, leading to bad performance for impoartant tasks. >> >> impoartant -> important >> >>> The MPAM driver needs a way to tell resctrl that no further configuration >>> should be attempted. >>> >>> Using resctrl_exit() for this leaves the system in a funny state as >>> resctrl is still mounted, but cannot be un-mounted because the sysfs >>> directory that is typically used has been removed. Dave Martin suggests >>> this may cause systemd trouble in the future as not all filesystems >>> can be unmounted. >>> >>> Add calls to remove all the files and directories in resctrl, and >>> remove the sysfs_remove_mount_point() call that leaves the system >>> in a funny state. When triggered, this causes all the resctrl files >>> to disappear. resctrl can be unmounted, but not mounted again. > >> I am not familiar with these flows so I would like to confirm ... >> In this scenario the resctrl filesystem will be unregistered, are >> you saying that it is possible to unmount a filesystem after it has >> been unregistered? > > Counter-intuitively: yes. > > The rules are described in fs/filesystems.c: We can access the members of the struct > file_system_type if the list lock is held, or a reference is held to the module. This is > how /proc/mounts is able to print the filesystem name from struct file_system_type without > taking the lock - it holds a reference to any module to prevent the structure from being hmmm ... does this mean I am supposed to find calls to try_module_get() in the flow from mounts_open_common()? > freed. Because resctrl can't be built as a module, we can say there is always a reference > held, and we can never free struct file_system_type. unregister_filesystem() continues to be called and as I understand in new MPAM usages will be called during runtime. unregister_filesystem() comments state "Once this function has returned the &struct file_system_type structure may be freed or reused.". Could you please highlight to me what gives the confidence of "we can say there is always a reference held"? Could you please point to me where that reference is obtained that will prevent the structure from being freed? Thank you Reinette