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Mon, 1 Jun 2026 15:40:52 +0000 Message-ID: <25890d7f-238c-4027-8ade-f30adb8f437e@intel.com> Date: Mon, 1 Jun 2026 08:40:49 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 8/9] x86/resctrl: Ensure domain fully initialized before placed on RCU list To: "Chen, Yu C" CC: , , , , , , , , , , , , , , References: <3ba2959b1cd3596e1e340eaee6b43487edaec0a4.1779476724.git.reinette.chatre@intel.com> <4053bd27-bca3-4745-9f34-65843e250f12@intel.com> <8d76c93f-7780-4fdd-a3b2-9faa8eeddf57@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: <8d76c93f-7780-4fdd-a3b2-9faa8eeddf57@intel.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4P221CA0009.NAMP221.PROD.OUTLOOK.COM (2603:10b6:303:8b::14) To SJ2PR11MB8370.namprd11.prod.outlook.com (2603:10b6:a03:540::20) 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: SJ2PR11MB8370:EE_|MW4PR11MB7030:EE_ X-MS-Office365-Filtering-Correlation-Id: 0274f94d-1aa5-4da2-4b98-08debff42957 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024|56012099006|4143699003|11063799006|6133799003|18002099003|22082099003; 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>>>       } >>>   -    list_add_tail_rcu(&d->hdr.list, add_pos); >>> - >>>       err = resctrl_online_mon_domain(r, &d->hdr); >>>       if (err) { >>> -        list_del_rcu(&d->hdr.list); >>> -        synchronize_rcu(); >>>           l3_mon_domain_free(hw_dom); >>> +        return; >>>       } >>> +    list_add_tail_rcu(&d->hdr.list, add_pos); >>>   } >>>     static void domain_add_cpu_mon(int cpu, struct rdt_resource *r) >> >> I resubmitted the last three patches of series to obtain Sashiko review [1] and >> respond to that feedback here: >> >> a) Sashiko: "Does this reordering expose the monitor directories to userspace before the >>     domain is actually added to the RCU list?" >> >>     Yes. As pointed out by Sashiko there is a short time where the monitoring data files >>     may be exposed to user space before the monitoring domain is added to the RCU list. >>     Also pointed out by Sashiko, if user attempts to read from such file it will return >>     -ENOENT. >> >>     This behavior looks correct and acceptable to me. > > It might not return -ENOENT because the read will be blocked by cpus_read_lock() > and after the reader has grabed the cpus_read_lock(), the reader will see the newly > added domain on the list: > > CPU 0 (writer)                            CPU 1 (reader) > ==========================                ========================== > cpus_write_lock held                      cpus_read_lock() **BLOCKED** >   mkdir mondata files >   list_add_tail_rcu() > cpus_write_unlock                         cpus_read_lock() returns >                                             lock(rdtgroup_mutex) >                                             resctrl_find_domain() **see a new complete domain** >                                             unlock(rdtgroup_mutex) >                                           cpus_read_unlock() Indeed. Thank you very much for the correction. > > > Reviewed-by: Chen Yu Thank you very much. Reinette