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Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=ktkhai@virtuozzo.com; Received: from [172.16.25.5] (195.214.232.6) by VI1PR0801MB1342.eurprd08.prod.outlook.com (2603:10a6:800:3a::28) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.841.15; Thu, 7 Jun 2018 18:54:59 +0000 Subject: Re: INFO: task hung in ip6gre_exit_batch_net To: Dmitry Vyukov Cc: syzbot , Christian Brauner , David Miller , David Ahern , Florian Westphal , Jiri Benc , LKML , Xin Long , mschiffer@universe-factory.net, netdev , syzkaller-bugs , Vladislav Yasevich References: <0000000000006e4595056dd23c16@google.com> From: Kirill Tkhai Message-ID: <04b6ee08-7919-bf2d-5b77-bd346a0bff48@virtuozzo.com> Date: Thu, 7 Jun 2018 21:54:55 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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linux-kernel@vger.kernel.org On 07.06.2018 21:23, Dmitry Vyukov wrote: > On Tue, Jun 5, 2018 at 3:55 PM, Kirill Tkhai wrote: >> On 05.06.2018 12:36, Dmitry Vyukov wrote: >>> On Tue, Jun 5, 2018 at 11:03 AM, Kirill Tkhai wrote: >>>> Hi, Dmirty! >>>> >>>> On 04.06.2018 18:22, Dmitry Vyukov wrote: >>>>> On Mon, Jun 4, 2018 at 5:03 PM, syzbot >>>>> wrote: >>>>>> Hello, >>>>>> >>>>>> syzbot found the following crash on: >>>>>> >>>>>> HEAD commit: bc2dbc5420e8 Merge branch 'akpm' (patches from Andrew) >>>>>> git tree: upstream >>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=164e42b7800000 >>>>>> kernel config: https://syzkaller.appspot.com/x/.config?x=982e2df1b9e60b02 >>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=bf78a74f82c1cf19069e >>>>>> compiler: gcc (GCC) 8.0.1 20180413 (experimental) >>>>>> >>>>>> Unfortunately, I don't have any reproducer for this crash yet. >>>>>> >>>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit: >>>>>> Reported-by: syzbot+bf78a74f82c1cf19069e@syzkaller.appspotmail.com >>>>> >>>>> Another hang on rtnl lock: >>>>> >>>>> #syz dup: INFO: task hung in netdev_run_todo >>>>> >>>>> May be related to "unregister_netdevice: waiting for DEV to become free": >>>>> https://syzkaller.appspot.com/bug?id=1a97a5bd119fd97995f752819fd87840ab9479a9 >>> >>> netdev_wait_allrefs does not hold rtnl lock during waiting, so it must >>> be something different. >>> >>> >>>>> Any other explanations for massive hangs on rtnl lock for minutes? >>>> >>>> To exclude the situation, when a task exists with rtnl_mutex held: >>>> >>>> would the pr_warn() from print_held_locks_bug() be included in the console output >>>> if they appear? >>> >>> Yes, everything containing "WARNING:" is detected as bug. >> >> OK, then dead task not releasing the lock is excluded. >> >> One more assumption: someone corrupted memory around rtnl_mutex and it looks like locked. >> (I track lockdep "(rtnl_mutex){+.+.}" prints in initial message as "nobody owns rtnl_mutex"). >> There may help a crash dump of the VM. > > I can't find any legend for these +'s and .'s, but {+.+.} is present > in large amounts in just any task hung report for different mutexes, > so I would not expect that it means corruption. > > Are dozens of known corruptions that syzkaller can trigger. But > usually they are reliably caught by KASAN. If any of them would lead > to silent memory corruption, we would got dozens of assorted crashes > throughout the kernel. We've seen that at some points, but not > recently. So I would assume that memory is not corrupted in all these > cases: > https://syzkaller.appspot.com/bug?id=2503c576cabb08d41812e732b390141f01a59545 This BUG clarifies the {+.+.}: 4 locks held by kworker/0:145/381: #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] work_static include/linux/workqueue.h:198 [inline] #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] set_work_data kernel/workqueue.c:619 [inline] #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] set_work_pool_and_clear_pending kernel/workqueue.c:646 [inline] #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] process_one_work+0xb12/0x1bb0 kernel/workqueue.c:2084 #1: ((work_completion)(&data->destroy_work)){+.+.}, at: [<00000000bbdd2115>] process_one_work+0xb89/0x1bb0 kernel/workqueue.c:2088 #2: (rtnl_mutex){+.+.}, at: [<000000009c9d14f8>] rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74 #3: (rcu_sched_state.exp_mutex){+.+.}, at: [<000000001ba1a807>] exp_funnel_lock kernel/rcu/tree_exp.h:272 [inline] #3: (rcu_sched_state.exp_mutex){+.+.}, at: [<000000001ba1a807>] _synchronize_rcu_expedited.constprop.72+0x9fa/0xac0 kernel/rcu/tree_exp.h:596 There we have rtnl_mutex locked and the {..} is like above. It's definitely locked since there is one more lock after it. This BUG happen because of there are many rtnl_mutex waiters while owner is synchronizing RCU. Rather clear for me in comparison to the topic's hung. > I wonder if it can be just that slow, but not actually hanged... net > namespace destruction is super slow, so perhaps under heavy load it > all stalls for minutes... Thanks, Kirill