From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f69.google.com (mail-pl0-f69.google.com [209.85.160.69]) by kanga.kvack.org (Postfix) with ESMTP id 334B76B0006 for ; Thu, 8 Feb 2018 09:18:34 -0500 (EST) Received: by mail-pl0-f69.google.com with SMTP id b34-v6so1721310plc.2 for ; Thu, 08 Feb 2018 06:18:34 -0800 (PST) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id a3sor22630pfe.133.2018.02.08.06.18.32 for (Google Transport Security); Thu, 08 Feb 2018 06:18:33 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20180208140833.lpr4yjn7g3v3cdy3@quack2.suse.cz> References: <001a11447070ac6fcb0564a08cb1@google.com> <20180207155229.GC10945@tassilo.jf.intel.com> <20180208092839.ebe5rk6mtvkk5da4@quack2.suse.cz> <20180208140833.lpr4yjn7g3v3cdy3@quack2.suse.cz> From: Dmitry Vyukov Date: Thu, 8 Feb 2018 15:18:11 +0100 Message-ID: Subject: Re: INFO: task hung in sync_blockdev Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Andi Kleen , syzbot , Andrew Morton , Andrey Ryabinin , jlayton@redhat.com, LKML , Linux-MM , Mel Gorman , Ingo Molnar , rgoldwyn@suse.com, syzkaller-bugs@googlegroups.com, linux-fsdevel@vger.kernel.org On Thu, Feb 8, 2018 at 3:08 PM, Jan Kara wrote: > On Thu 08-02-18 14:28:08, Dmitry Vyukov wrote: >> On Thu, Feb 8, 2018 at 10:28 AM, Jan Kara wrote: >> > On Wed 07-02-18 07:52:29, Andi Kleen wrote: >> >> > #0: (&bdev->bd_mutex){+.+.}, at: [<0000000040269370>] >> >> > __blkdev_put+0xbc/0x7f0 fs/block_dev.c:1757 >> >> > 1 lock held by blkid/19199: >> >> > #0: (&bdev->bd_mutex){+.+.}, at: [<00000000b4dcaa18>] >> >> > __blkdev_get+0x158/0x10e0 fs/block_dev.c:1439 >> >> > #1: (&ldata->atomic_read_lock){+.+.}, at: [<0000000033edf9f2>] >> >> > n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131 >> >> > 1 lock held by syz-executor5/19330: >> >> > #0: (&bdev->bd_mutex){+.+.}, at: [<00000000b4dcaa18>] >> >> > __blkdev_get+0x158/0x10e0 fs/block_dev.c:1439 >> >> > 1 lock held by syz-executor5/19331: >> >> > #0: (&bdev->bd_mutex){+.+.}, at: [<00000000b4dcaa18>] >> >> > __blkdev_get+0x158/0x10e0 fs/block_dev.c:1439 >> >> >> >> It seems multiple processes deadlocked on the bd_mutex. >> >> Unfortunately there's no backtrace for the lock acquisitions, >> >> so it's hard to see the exact sequence. >> > >> > Well, all in the report points to a situation where some IO was submitted >> > to the block device and never completed (more exactly it took longer than >> > those 120s to complete that IO). It would need more digging into the >> > syzkaller program to find out what kind of device that was and possibly why >> > the IO took so long to complete... >> >> >> Would a traceback of all task stacks help in this case? >> What I've seen in several "task hung" reports is that the CPU >> traceback is not showing anything useful. So perhaps it should be >> changed to task traceback? Or it would not help either? > > Task stack traceback for all tasks (usually only tasks in D state - i.e. > sysrq-w - are enough actually) would definitely help for debugging > deadlocks on sleeping locks. For this particular case I'm not sure if it > would help or not since it is quite possible the IO is just sitting in some > queue never getting processed That's what I was afraid of. > due to some racing syzkaller process tearing > down the device in the wrong moment or something like that... Such case is > very difficult to debug without full kernel crashdump of the hung kernel > (or a reproducer for that matter) and even with that it is usually rather > time consuming. But for the deadlocks which do occur more frequently it > would be probably worth the time so it would be nice if such option was > eventually available. By "full kernel crashdump" you mean kdump thing, or something else? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org