linux-kernel - Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also an unrelated ext4 data loss bug)

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Date:	Tue, 23 Oct 2012 16:56:00 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	Nix <nix@...eri.org.uk>, "Ted Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Schumaker, Bryan" <Bryan.Schumaker@...app.com>,
	Peng Tao <bergwolf@...il.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>
Subject: Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression?
 (also an unrelated ext4 data loss bug)

On Tue, 2012-10-23 at 12:46 -0400, J. Bruce Fields wrote:
> On Tue, Oct 23, 2012 at 05:32:21PM +0100, Nix wrote:
> > On 23 Oct 2012, J. Bruce Fields uttered the following:
> > > nfs-utils shouldn't be capable of oopsing the kernel, so from my
> > > (selfish) point of view I'd actually rather you stick with whatever you
> > > have and try to reproduce the oops.
> > 
> > Reproduced in 3.6.3, not in 3.6.1, not tried 3.6.2. Capturing it was
> > rendered somewhat difficult by an ext4/JBD2 bug which leads to data loss
> > in /var on every reboot out of 3.6.1 and on some reboots out of 3.6.3 (I
> > have runs of NULs in my logs now, which keep eating the oopses):
> > 
> > [while in 3.6.1]
> > [   88.565698] JBD2: Spotted dirty metadata buffer (dev = dm-5, blocknr = 0). There's a risk of filesystem corruption in case of system crash.
> > [   88.799263] JBD2: Spotted dirty metadata buffer (dev = dm-5, blocknr = 0). There's a risk of filesystem corruption in case of system crash.
> > [   89.648152] ------------[ cut here ]------------
> > [   89.648386] WARNING: at fs/inode.c:280 drop_nlink+0x25/0x42()
> > [   89.648614] Hardware name: empty
> > [   89.648833] Modules linked in: firewire_ohci firewire_core [last unloaded: microcode]
> > [   89.649382] Pid: 1484, comm: dhcpd Not tainted 3.6.1-dirty #1
> > [   89.649610] Call Trace:
> > [   89.649832]  [<ffffffff810608c0>] warn_slowpath_common+0x83/0x9b
> > [   89.650063]  [<ffffffff810608f2>] warn_slowpath_null+0x1a/0x1c
> > [   89.650292]  [<ffffffff8112efbf>] drop_nlink+0x25/0x42
> > [   89.650533]  [<ffffffff81187112>] ext4_dec_count+0x26/0x28
> > [   89.650763]  [<ffffffff8118abb8>] ext4_rename+0x4ec/0x7b4
> > [   89.650993]  [<ffffffff81125d81>] ? vfs_rename+0xbe/0x3b7
> > [   89.651224]  [<ffffffff81125f3f>] vfs_rename+0x27c/0x3b7
> > [   89.651454]  [<ffffffff81127a53>] sys_renameat+0x1b1/0x228
> > [   89.651682]  [<ffffffff8114bda1>] ? fsnotify+0x226/0x249
> > [   89.651911]  [<ffffffff81239b75>] ? security_inode_permission+0x1e/0x20
> > [   89.652145]  [<ffffffff8111a240>] ? vfs_write+0x116/0x142
> > [   89.652372]  [<ffffffff81127ae5>] sys_rename+0x1b/0x1e
> > [   89.652601]  [<ffffffff814fffa2>] system_call_fastpath+0x16/0x1b
> > [...]
> > [while having just booted into 3.6.1 after some time in 3.6.3: the FS
> >  was clean, and fscked on the previous boot into 3.6.3 after a previous
> >  instance of this bug]
> > Oct 23 17:18:26 spindle crit: [   67.625319] EXT4-fs error (device dm-5): mb_free_blocks:1300: group 65, block 2143748:freeing already freed block (bit 13828)
> > 
> > This may well be a 3.6.1-specific bug fixed in 3.6.3, but it's hard to
> > tell since most of my reboots are 3.6.1->3.6.3 or vice versa right now.
> > 
> > 
> > Anyway, here's the NFSv4 oops (not a panic: it helps if I remember to
> > turn off panic_on_oops when I come home from the holidays).
> > 
> > It's a lockd problem, and probably happens during delivery of mail over
> > NFS (my mailserver load soars when it happens):
> > 
> > [  813.110354] ------------[ cut here ]------------
> > [  813.110585] kernel BUG at fs/lockd/mon.c:150!
> 
> So nsm_mon_unmon() is being passed a NULL client.
> 
> There are three container patches between 3.6.1 and 3.6.3:
> 
> 	lockd: per-net NSM client creation and destruction helpers introduced
> 	lockd: use rpc client's cl_nodename for id encoding
> 	lockd: create and use per-net NSM RPC clients on MON/UNMON requests
> 
> and that last does change nsm_monitor's call to nsm_mon_unmon, so that's
> almost certainly it....
> 
> Looks like there's some confusion about whether nsm_client_get() returns
> NULL or an error?

nsm_client_get() looks extremely racy in the case where ln->nsm_users ==
0.

Since we never recheck the value of ln->nsm_users after taking
nsm_create_mutex, what is stopping 2 different threads from both setting
ln->nsm_clnt and re-initialising ln->nsm_users?


-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com