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McKenney" , Boqun Feng , neeraj.upadhyay@kernel.org, urezki@gmail.com, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, xiqi2@huawei.com, "Wangshaobo (bobo)" References: <9acd5f9f-6732-7701-6880-4b51190aa070@huawei.com> <3ce6f3ce-5dfb-8c59-cb7b-4619b70f8d25@huawei.com> <20250603185939.GA1109523@joelnvbox> <066e8121-c6c5-48ac-b35a-e6430d986dff@nvidia.com> Content-Language: en-US In-Reply-To: <066e8121-c6c5-48ac-b35a-e6430d986dff@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MN2PR11CA0013.namprd11.prod.outlook.com (2603:10b6:208:23b::18) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN7PR12MB8059:EE_|IA0PR12MB8648:EE_ X-MS-Office365-Filtering-Correlation-Id: 7ab65855-d0e5-424d-ab20-08dda2d404a3 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|376014|366016|7053199007; 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The Calltrace is too long. I put it in the end. >>>>> The issue can also be reproduced in the latest kernel. >>>>> >>>>> The issue is as follows. CPU3 is waiting for a spin_lock, which is got by CPU1. >>>>> But CPU1 stuck in the following dead loop. >>>>> >>>>> irq_exit() >>>>> __irq_exit_rcu() >>>>> /* in_hardirq() returns false after this */ >>>>> preempt_count_sub(HARDIRQ_OFFSET) >>>>> tick_irq_exit() >>>>> tick_nohz_irq_exit() >>>>> tick_nohz_stop_sched_tick() >>>>> trace_tick_stop() /* a bpf prog is hooked on this trace point */ >>>>> __bpf_trace_tick_stop() >>>>> bpf_trace_run2() >>>>> rcu_read_unlock_special() >>>>> /* will send a IPI to itself */ >>>>> irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); >>>>> >>>>> /* after interrupt is enabled again, the irq_work is called */ >>>>> asm_sysvec_irq_work() >>>>> sysvec_irq_work() >>>>> irq_exit() /* after handled the irq_work, we again enter into irq_exit() */ >>>>> __irq_exit_rcu() >>>>> ...skip... >>>>> /* we queue a irq_work again, and enter a dead loop */ >>>>> irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); >>>> >>>> This seems legitimate, Boqun and I were just talking about it. He may >>>> share more thoughts but here are a few: >>>> >>>> Maybe we can delay subsequent clearing of the flag in >>>> rcu_preempt_deferred_qs_handler() using a timer and an exponential >>>> back-off? That way we are not sending too many self-IPIs. >>>> >>>> And reset the process at the end of a grace period. >>>> >>>> Or just don't send subsequent self-IPIs if we just sent one for the >>>> rdp. Chances are, if we did not get the scheduler's attention during >>>> the first one, we may not in subsequent ones I think. Plus we do send >>>> other IPIs already if the grace period was over extended (from the FQS >>>> loop), maybe we can tweak that? >>> >>> Thanks a lot for your reply. I think it's hard for me to fix this issue as >>> above without introducing new bugs. I barely understand the RCU code. But I'm >>> very glad to help test if you have any code modifiction need to. I have >>> the VM and the syskaller benchmark which can reproduce the problem. >> >> Sure, I understand. This is already incredibly valuable so thank you again. >> Will request for your testing help soon. I also have a test module now which >> can sort-off reproduce this. Keep you posted! > > Oh sorry I meant to ask - could you provide the full kernel log and also is > there a standalone reproducer syzcaller binary one can run to reproduce it in a VM? > Sorry for the noise, but please provide the full .config as well. I am curious if you have CONFIG_RCU_STRICT_GRACE_PERIOD. Since that has an effect on rcu_read_unlock_special(). Thanks! - Joel