linux-kernel - Re: Kernel BUG when syncing ext2 if USB stick is removed

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Date:	Thu, 12 May 2011 16:59:16 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Torsten Hilbrich <torsten.hilbrich@...unet.com>,
	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Maciej Rutecki <maciej.rutecki@...il.com>
Subject: Re: Kernel BUG when syncing ext2 if USB stick is removed

On Thu, 2011-05-12 at 14:42 -0700, Andrew Morton wrote:
> On Thu, 12 May 2011 16:54:40 +0200
> Torsten Hilbrich <torsten.hilbrich@...unet.com> wrote:
> 
> > The error can be reproduced in both linux-2.6 master
> > (3568bd9720b4a775f28a718fcbb462ce2f386988) and v2.6.38.6. It cannot be
> > reproduced in v2.6.38.5, because the error occurs only after:
> > 
> > commit 1f74c190e1e97a38823c07fdc71780580a0fc03f
> > Author: James Bottomley <James.Bottomley@...e.de>
> > Date:   Fri Apr 22 10:39:59 2011 -0500
> > 
> >     put stricter guards on queue dead checks
> > 
> >     commit 86cbfb5607d4b81b1a993ff689bbd2addd5d3a9b upstream.
> > 
> > in the 2.6.38 stable line.
> 
> A 2.6.38.5 -> 2.6.38.6 regression is presumably also a
> 2.6.38->2.6.39-rc regression.
> 
> > Here is the error message for master:
> > 
> > general protection fault: 0000 [#1] SMP
> > last sysfs file:
> > CPU 1
> > Modules linked in:
> > 
> > Pid: 1926, comm: sync Not tainted 2.6.39-rc7+ #39 LENOVO 20077KG/20077KG
> > RIP: 0010:[<ffffffff811231bf>]  [<ffffffff811231bf>]
> > __mark_inode_dirty+0x14f/0x200
> > RSP: 0018:ffff88007bebbe08  EFLAGS: 00010246
> > RAX: ffff88007d031470 RBX: ffff88007d031408 RCX: ffff88007d031470
> > RDX: 6b6b6b6b6b6b6b6b RSI: ffffffff81d33f2a RDI: ffffffff82002300
> > RBP: ffff88007bebbe28 R08: 0000000000000000 R09: 0000000000000000
> > R10: ffff88007bde3d68 R11: ffff88007c2fba6f R12: ffff88007c350300
> > R13: ffff88007c350458 R14: 0000000000000000 R15: ffffffff81127e30
> > FS:  00007f8306063700(0000) GS:ffff88007fd00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> > CR2: 0000000001f40600 CR3: 000000007b968000 CR4: 00000000000006a0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> > Process sync (pid: 1926, threadinfo ffff88007beba000, task ffff88007c610820)
> > Stack:
> >  ffff88007d031550 ffffea0000082638 0000000000000000 0000000000000000
> >  ffff88007bebbe58 ffffffff8112a5ff ffffffff81d3c8d8 ffff88007d223eb0
> >  ffff880002541400 ffff88007d223eb0 ffff88007bebbe78 ffffffff8112a6b6
> > Call Trace:
> >  [<ffffffff8112a5ff>] __set_page_dirty+0x6f/0xc0
> >  [<ffffffff8112a6b6>] mark_buffer_dirty+0x66/0xa0
> >  [<ffffffff8117c3ce>] ext2_sync_super+0x8e/0xf0
> >  [<ffffffff8117c495>] ext2_sync_fs+0x65/0x80
> >  [<ffffffff81127dfe>] __sync_filesystem+0x5e/0x90
> >  [<ffffffff81127e4f>] sync_one_sb+0x1f/0x30
> >  [<ffffffff81100021>] iterate_supers+0x71/0xd0
> >  [<ffffffff81127e8f>] sys_sync+0x2f/0x70
> >  [<ffffffff819c69ab>] system_call_fastpath+0x16/0x1b
> > Code: e8 67 be 89 00 48 8b 05 20 47 ff 00 48 8b 53 70 48 8b 4b 68 48 89
> > 43 50 48 8d 43 68 48 89 51 08 48 89 0a 49 8b 94 24 58 01 00 00
> >  89 42 08 48 89 53 68 4c 89 6b 70 49 89 84 24 58 01 00 00 fe
> > RIP  [<ffffffff811231bf>] __mark_inode_dirty+0x14f/0x200
> >  RSP <ffff88007bebbe08>
> > ---[ end trace 04d7660d6043ca51 ]---
> > 
> > If I parsed the error location correctly, it is the:
> > 
> >   next->prev = prev; // mov %rcx,(%rdx)
> > 
> > statement from __list_del called via __list_del_entry, list_move from
> > the line:
> > 
> >   list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
> > 
> > in __mark_inode_dirty. rdx seems to be
> > 
> > Here are the steps for reproduction:
> > 
> > 1. mount an USB stick with ext2 FS (mount /dev/sdb1 /mnt)
> > 2. open file on USB stick for writing (cat > /mnt/foo)
> > 3. press some return
> > 4. Remove USB stick
> > 5. In another console run sync
> > 
> > I will append the full kernel log (screenlog.0) and the configuration
> > (config-master) to this mail.
> 
> hm, maybe *bdi was freed at this time.  But if so I'd have expected
> __mark_inode_dirty() to crash earlier than in the list_move().
> 
> James, the cure looks worse than the disease - should we revert "put
> stricter guards on queue dead checks" for now?

Well, I only accelerated the patch because it got marked as a
regression, so it's now regression if we do or if we don't.

This one actually doesn't look like a direct consequence, if it's list
corruption.  If the bdi got prematurely freed, then there's a ref
counting error in our model somewhere and the sdev patch just exposed
it.  Since it should be easily reproducible, I'll see if I can track it
down.

James


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