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Thu, 18 Mar 2021 07:32:08 -0700 (PDT) To: Hillf Danton , Ming Lei Cc: syzbot , axboe@kernel.dk, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com References: <0000000000006e9e0705bd91f762@google.com> <20210318083340.1900-1-hdanton@sina.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [syzbot] WARNING in __percpu_ref_exit (2) Message-ID: <79ce5bcd-b314-dded-0b36-0a8fb66f5a7a@gmail.com> Date: Thu, 18 Mar 2021 14:28:11 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <20210318083340.1900-1-hdanton@sina.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 18/03/2021 08:33, Hillf Danton wrote: > On Mon, 15 Mar 2021 12:18:20 +0000 Pavel Begunkov wrote: >> On 15/03/2021 11:58, syzbot wrote: >>> Hello, >>> >>> syzbot found the following issue on: >>> >>> HEAD commit: 75013c6c Merge tag 'perf_urgent_for_v5.12-rc3' of git://gi.. >>> git tree: upstream >>> console output: https://syzkaller.appspot.com/x/log.txt?x=174df32ad00000 >>> kernel config: https://syzkaller.appspot.com/x/.config?x=844457676c06b88c >>> dashboard link: https://syzkaller.appspot.com/bug?extid=d6218cb2fae0b2411e9d >>> userspace arch: i386 >>> >>> Unfortunately, I don't have any reproducer for this issue yet. >>> >>> IMPORTANT: if you fix the issue, please add the following tag to the commit: >>> Reported-by: syzbot+d6218cb2fae0b2411e9d@syzkaller.appspotmail.com >>> ------------[ cut here ]------------ >>> WARNING: CPU: 1 PID: 53 at lib/percpu-refcount.c:113 __percpu_ref_exit+0x98/0x100 lib/percpu-refcount.c:113 >> >> if (percpu_count) { >> /* non-NULL confirm_switch indicates switching in progress */ >> WARN_ON_ONCE(ref->data && ref->data->confirm_switch); >> ... >> } >> >> Points to this warning. Not sure, but not yet included >> "io_uring: halt SQO submission on ctx exit" may fix it or at least is >> related. > > Seems it does not, nor related, see below. >> >>> Modules linked in: >>> CPU: 1 PID: 53 Comm: kworker/u4:2 Not tainted 5.12.0-rc2-syzkaller #0 >>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 >>> Workqueue: events_unbound io_ring_exit_work >>> RIP: 0010:__percpu_ref_exit+0x98/0x100 lib/percpu-refcount.c:113 >>> Code: fd 49 8d 7c 24 10 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 75 61 49 83 7c 24 10 00 74 07 e8 28 42 ac fd <0f> 0b e8 21 42 ac fd 48 89 ef e8 e9 fa da fd 48 89 da 48 b8 00 00 >>> RSP: 0018:ffffc90000f1fb78 EFLAGS: 00010293 >>> RAX: 0000000000000000 RBX: ffff88805c976000 RCX: 0000000000000000 >>> RDX: ffff888011839bc0 RSI: ffffffff83c76be8 RDI: ffff88802b2a9010 >>> RBP: 0000607f46077778 R08: 0000000000000000 R09: ffffffff8fab0967 >>> R10: ffffffff83c76b88 R11: 0000000000000009 R12: ffff88802b2a9000 >>> R13: 0000000000000001 R14: ffff88802b2a9000 R15: dffffc0000000000 >>> FS: 0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000 >>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>> CR2: 00000000085a0004 CR3: 000000001896a000 CR4: 00000000001506f0 >>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>> Call Trace: >>> percpu_ref_exit+0x3b/0x140 lib/percpu-refcount.c:134 >>> io_ring_ctx_free fs/io_uring.c:8419 [inline] >>> io_ring_exit_work+0x599/0xcf0 fs/io_uring.c:8565 >>> process_one_work+0x98d/0x1600 kernel/workqueue.c:2275 >>> worker_thread+0x64c/0x1120 kernel/workqueue.c:2421 >>> kthread+0x3b1/0x4a0 kernel/kthread.c:292 >>> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294 >>> >>> >>> --- >>> This report is generated by a bot. It may contain errors. >>> See https://goo.gl/tpsmEJ for more information about syzbot. >>> syzbot engineers can be reached at syzkaller@googlegroups.com. >>> >>> syzbot will keep track of this issue. See: >>> https://goo.gl/tpsmEJ#status for how to communicate with syzbot. >>> >> >> -- >> Pavel Begunkov > > Thoughts for sync RCU are appreciated if the chance for the race > between rcu and workqueue is not zero on killing io ctx. Would you elaborate? Because your case below doesn't make much sense. 1) io_ring_ctx_wait_and_kill() indeed kills ctx->refs 2) io_ring_exit_work() waits for a completion signaled by ctx->refs hitting 0 3) and only then calls io_ring_ctx_free(). And 2) won't complete until the switching is over > > CPU0 > ---- > io_ring_ctx_wait_and_kill > percpu_ref_kill(&ctx->refs); > percpu_ref_kill_and_confirm(ref, NULL); > spin_lock_irqsave(&percpu_ref_switch_lock, flags); > __percpu_ref_switch_mode(ref, confirm_switch); > __percpu_ref_switch_to_atomic > ref->data->confirm_switch = confirm_switch ?: > percpu_ref_noop_confirm_switch; > call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu); > spin_unlock_irqrestore(&percpu_ref_switch_lock, flags); > > INIT_WORK(&ctx->exit_work, io_ring_exit_work); > queue_work(system_unbound_wq, &ctx->exit_work); > > CPU1 > ---- > io_ring_exit_work > io_ring_ctx_free(ctx); > percpu_ref_exit(&ctx->refs); > __percpu_ref_exit(ref); > WARN_ON_ONCE(ref->data && > ref->data->confirm_switch); > > > percpu_ref_switch_to_atomic_rcu > percpu_ref_call_confirm_rcu(rcu); > data->confirm_switch(ref); > data->confirm_switch = NULL; > wake_up_all(&percpu_ref_switch_waitq); > -- Pavel Begunkov