From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ns211617.ip-188-165-215.eu ([188.165.215.42]:43060 "EHLO mx.speed47.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932340AbbIVHeW (ORCPT ); Tue, 22 Sep 2015 03:34:22 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Date: Tue, 22 Sep 2015 09:34:19 +0200 From: =?UTF-8?Q?St=C3=A9phane_Lesimple?= To: Qu Wenruo Cc: Qu Wenruo , linux-btrfs@vger.kernel.org Subject: Re: kernel BUG at linux-4.2.0/fs/btrfs/extent-tree.c:1833 on rebalance In-Reply-To: <5600B0BF.604@cn.fujitsu.com> References: <9c864637fe7676a8b7badc5ddd7a4e0c@all.all> <55F9486F.4040302@googlemail.com> <0973de930ee87e102c533c719807b748@all.all> <55FA2D9A.1060405@cn.fujitsu.com> <55FA60C5.5090002@cn.fujitsu.com> <7a6f2d794fb6cbf7d598b92e3470201c@all.all> <55FA759E.6030707@cn.fujitsu.com> <3386a8bfa1a5796460306a53a668e47e@all.all> <55FA98D8.5010301@gmx.com> <53a5553a9c5301789e246144bb264e43@all.all> <55FB61E9.4000300@cn.fujitsu.com> <2ce9b35f73732b145e0f80b18f230a52@all.all> <762ec73d5389b5057be4d3c17f74e1f9@all.all> <55FE0A50.9060607@gmx.com> <3ba27cf5afd82cf4e3bde718386b7cc3@all.all> <55FE8FB6.4070509@gmx.com> <72b4368e7180a4d703ef3ea1112d7358@all.all> <4749d42363070fcd228af172781750df@all.all> <5600B0BF.604@cn.fujitsu.com> Message-ID: <0a4be8fab4876a245900e4833e8139e0@all.all> Sender: linux-btrfs-owner@vger.kernel.org List-ID: Le 2015-09-22 03:37, Qu Wenruo a écrit : > Stéphane Lesimple wrote on 2015/09/22 03:30 +0200: >> Le 2015-09-20 13:14, Stéphane Lesimple a écrit : >>> Le 2015-09-20 12:51, Qu Wenruo a écrit : >>>>>> Would you please use gdb to show the codes of >>>>>> "btrfs_qgroup_rescan_worker+0x388" ? >>>>>> (Need kernel debuginfo) >>>>>> >>>>>> My guess is the following line:(pretty sure, but not 100% sure) >>>>>> ------ >>>>>> /* >>>>>> * only update status, since the previous part has alreay >>>>>> updated the >>>>>> * qgroup info. >>>>>> */ >>>>>> trans = btrfs_start_transaction(fs_info->quota_root, 1); >>>>>> <<<<< >>>>>> if (IS_ERR(trans)) { >>>>>> err = PTR_ERR(trans); >>>>>> btrfs_err(fs_info, >>>>>> "fail to start transaction for status >>>>>> update: %d\n", >>>>>> err); >>>>>> goto done; >>>>>> } >>>>>> ------ >>>>> >>>>> The kernel and modules were already compiled with debuginfo. >>>>> However for some reason, I couldn't get gdb disassembly of >>>>> /proc/kcore >>>>> properly >>>>> aligned with the source I compiled: the asm code doesn't match the >>>>> C >>>>> code shown >>>>> by gdb. In any case, watching the source of this function, this is >>>>> the >>>>> only place >>>>> btrfs_start_transaction is called, so we can be 100% sure it's >>>>> where >>>>> the >>>>> crash >>>>> happens indeed. >>>> >>>> Yep, that's the only caller. >>>> >>>> Here is some useful small hint to locate the code, if you are >>>> interestied in kernel development. >>>> >>>> # Not sure about whether ubuntu gzipped modules, at least Arch does >>>> # compress it >>>> $ cp /kernel/fs/btrfs/btrfs.ko.gz /tmp/ >>>> $ gunzip /tmp/btrfs.ko.gz >>>> $ gdb /tmp/btrfs.ko >>>> # Make sure gdb read all the needed debuginfo >>>> $ gdb list *(btrfs_qgroup_rescan_worker+0x388) >>>> >>>> And gdb will find the code position for you. >>>> Quite easy one, only backtrace info is needed. >>> >>> Ah, thanks for the tips, I was loading whole vmlinux and using >>> /proc/kcore >>> as the core info, then adding the module with "add-symbol-file". But >>> as >>> we're just looking for the code and not the variables, it was indeed >>> completely overkill. >>> >>> (gdb) list *(btrfs_qgroup_rescan_worker+0x388) >>> 0x98068 is in btrfs_qgroup_rescan_worker (fs/btrfs/qgroup.c:2328). >>> 2323 >>> 2324 /* >>> 2325 * only update status, since the previous part has >>> alreay updated the >>> 2326 * qgroup info. >>> 2327 */ >>> 2328 trans = btrfs_start_transaction(fs_info->quota_root, >>> 1); >>> 2329 if (IS_ERR(trans)) { >>> 2330 err = PTR_ERR(trans); >>> 2331 btrfs_err(fs_info, >>> 2332 "fail to start transaction for >>> status update: %d\n", >>> >>> So this just confirms what we were already 99% sure of. >>> >>>> Another hint is about how to collect the kernel crash info. >>>> Your netconsole setup would be definitely one good practice. >>>> >>>> Another one I use to collect crash info is kdump. >>>> Ubuntu should have a good wiki on it. >>> >>> I've already come across kdump a few times, but never really look >>> into >>> it. >>> To debug the other complicated extend backref bug, it could be of >>> some >>> use. >>> >>>>>>>> So, as a quick summary of this big thread, it seems I've been >>>>>>>> hitting >>>>>>>> 3 bugs, all reproductible : >>>>>>>> - kernel BUG on balance (this original thread) >>>>>> >>>>>> For this, I can't provide much help, as extent backref bug is >>>>>> quite >>>>>> hard to debug, unless a developer is interested in it and find a >>>>>> stable way to reproduce it. >>>>> >>>>> Yes, unfortunately as it looks so much like a race condition, I >>>>> know >>>>> I can >>>>> reproduce it with my worflow, but it can take between 1 minute and >>>>> 12 >>>>> hours, >>>>> so I wouldn't call it a "stable way" to reproduce it unfortunately >>>>> :( >>>>> >>>>> Still if any dev is interested in it, I can reproduce it, with a >>>>> patched >>>>> kernel if needed. >>>> >>>> Maybe you are already doing it, you can only compile the btrfs >>>> modules, which will be far more faster than compile the whole >>>> kernel, >>>> if and only if the compiled module can be loaded. >>> >>> Yes, I've compiled this 4.3.0-rc1 in a completely modular form, so >>> I'll try to >>> load the modified module and see if the running kernel accepts it. I >>> have to rmmod >>> the loaded module first, hence umounting any btrfs fs before that. >>> Should be able >>> to do it in a couple hours. >>> >>> I'll delete again all my snapshots and run my script. Should be easy >>> to trigger >>> the (hopefully worked-around) bug again. >> >> Well, I didn't trigger this exact bug, but another one, not less >> severe >> though, as it also crashed the system: >> >> [92098.841309] general protection fault: 0000 [#1] SMP >> [92098.841338] Modules linked in: ... >> [92098.841814] CPU: 1 PID: 24655 Comm: kworker/u4:12 Not tainted >> 4.3.0-rc1 #1 >> [92098.841834] Hardware name: ASUS All Series/H87I-PLUS, BIOS 1005 >> 01/06/2014 >> [92098.841868] Workqueue: btrfs-qgroup-rescan >> btrfs_qgroup_rescan_helper >> [btrfs] >> [92098.841889] task: ffff8800b6cc4100 ti: ffff8800a3dc8000 task.ti: >> ffff8800a3dc8000 >> [92098.841910] RIP: 0010:[] [] >> memcpy_erms+0x6/0x10 >> [92098.841935] RSP: 0018:ffff8800a3dcbcc8 EFLAGS: 00010207 >> [92098.841950] RAX: ffff8800a3dcbd67 RBX: 0000000000000009 RCX: >> 0000000000000009 >> [92098.841970] RDX: 0000000000000009 RSI: 0005080000000000 RDI: >> ffff8800a3dcbd67 >> [92098.841989] RBP: ffff8800a3dcbd00 R08: 0000000000019c60 R09: >> ffff88011fb19c60 >> [92098.842009] R10: ffffea0003006480 R11: 0000000001000000 R12: >> ffff8800b76c32c0 >> [92098.842028] R13: 0000160000000000 R14: ffff8800a3dcbd70 R15: >> 0000000000000009 >> [92098.842048] FS: 0000000000000000(0000) GS:ffff88011fb00000(0000) >> knlGS:0000000000000000 >> [92098.842070] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> [92098.842086] CR2: 00007fe1f2bd8000 CR3: 0000000001c10000 CR4: >> 00000000000406e0 >> [92098.842105] Stack: >> [92098.842111] ffffffffc035a5d8 ffffffffc0396d00 000000000000028b >> 0000000000000000 >> [92098.842212] 0000cc6c00000000 ffff8800b76c3200 0000160000000000 >> ffff8800a3dcbdc0 >> [92098.842237] ffffffffc039af3d ffff8800c7196dc8 ffff8800c7196e08 >> ffff8800c7196da0 >> [92098.842261] Call Trace: >> [92098.842277] [] ? read_extent_buffer+0xb8/0x110 >> [btrfs] >> [92098.842304] [] ? btrfs_find_all_roots+0x60/0x70 >> [btrfs] >> [92098.842329] [] >> btrfs_qgroup_rescan_worker+0x28d/0x5a0 [btrfs] > > Would you please show the code of it? > This one seems to be another stupid bug I made when rewriting the > framework. > Maybe I forgot to reinit some variants or I'm screwing memory... (gdb) list *(btrfs_qgroup_rescan_worker+0x28d) 0x97f6d is in btrfs_qgroup_rescan_worker (fs/btrfs/ctree.h:2760). 2755 2756 static inline void btrfs_disk_key_to_cpu(struct btrfs_key *cpu, 2757 struct btrfs_disk_key *disk) 2758 { 2759 cpu->offset = le64_to_cpu(disk->offset); 2760 cpu->type = disk->type; 2761 cpu->objectid = le64_to_cpu(disk->objectid); 2762 } 2763 2764 static inline void btrfs_cpu_key_to_disk(struct btrfs_disk_key *disk, (gdb) Does it makes sense ? >> [92098.842351] [] ? >> ttwu_do_activate.constprop.90+0x5d/0x70 >> [92098.842377] [] normal_work_helper+0xc0/0x270 >> [btrfs] >> [92098.842401] [] >> btrfs_qgroup_rescan_helper+0x12/0x20 [btrfs] >> [92098.842421] [] process_one_work+0x14e/0x3d0 >> [92098.842438] [] worker_thread+0x11a/0x470 >> [92098.842454] [] ? rescuer_thread+0x310/0x310 >> [92098.842471] [] kthread+0xc9/0xe0 >> [92098.842485] [] ? kthread_park+0x60/0x60 >> [92098.842502] [] ret_from_fork+0x3f/0x70 >> [92098.842517] [] ? kthread_park+0x60/0x60 >> [92098.842532] Code: ff eb eb 90 90 eb 1e 0f 1f 00 48 89 f8 48 89 d1 >> 48 >> c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 c3 66 0f 1f 44 00 00 48 89 f8 >> 48 >> 89 d1 a4 c3 0f 1f 80 00 00 00 00 48 89 f8 48 83 fa 20 72 7e 40 38 >> [92098.842658] RIP [] memcpy_erms+0x6/0x10 >> [92098.842675] RSP >> [92098.849594] ---[ end trace 9d5fb7931a3ec713 ]--- >> >> I would definitely say that rescans should be avoided on current >> kernels >> as the possibility that it'll bring the system down shouldn't be >> ignored. >> It confirms that this code really needs a rewrite ! >> >> Regards, >> > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" > in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html