Re: hang in writeback code on nfsv4 mount

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Aug 25, 2010 at 09:32:25AM +0300, Artem Bityutskiy wrote:
> On Wed, 2010-08-25 at 04:34 +0200, ext J. Bruce Fields wrote:
> > As of 253c34e9b10c30d3064be654b5b78fbc1a8b1896 "writeback: prevent
> > unnecessary bdi threads wakeups", any nfs mount hangs for me.  Is this a
> > known issue?
> > 
> > --b.
> > 
> > INFO: task mount.nfs4:3812 blocked for more than 120 seconds.
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > mount.nfs4    D 0000000000000000  2880  3812   3811 0x00000000
> >  ffff88001ed25a28 0000000000000046 ffff88001ed25fd8 ffff88001ed25fd8
> >  ffff88001ed24000 ffff88001ed24000 ffff88001ed24000 ffff88001f9503a0
> >  ffff88001ed25fd8 ffff88001f9503a8 ffff88001ed24000 ffff88001ed25fd8
> > Call Trace:
> >  [<ffffffff819608dd>] schedule_timeout+0x1cd/0x2e0
> >  [<ffffffff8106a23c>] ? mark_held_locks+0x6c/0xa0
> >  [<ffffffff81963970>] ? _raw_spin_unlock_irq+0x30/0x60
> >  [<ffffffff8106a51d>] ? trace_hardirqs_on_caller+0x14d/0x190
> >  [<ffffffff819671fe>] ? sub_preempt_count+0xe/0xd0
> >  [<ffffffff8195fc50>] wait_for_common+0x120/0x190
> >  [<ffffffff81033bb0>] ? default_wake_function+0x0/0x20
> >  [<ffffffff8195fd9d>] wait_for_completion+0x1d/0x20
> >  [<ffffffff8105951a>] kthread_stop+0x4a/0x150
> >  [<ffffffff81061980>] ? thaw_process+0x70/0x80
> >  [<ffffffff810cc5aa>] bdi_unregister+0x10a/0x1a0
> >  [<ffffffff81229cd9>] nfs_put_super+0x19/0x20
> >  [<ffffffff810ee7d4>] generic_shutdown_super+0x54/0xe0
> >  [<ffffffff810ee8c6>] kill_anon_super+0x16/0x60
> >  [<ffffffff8122d2c9>] nfs4_kill_super+0x39/0x90
> >  [<ffffffff810ed955>] deactivate_locked_super+0x45/0x60
> >  [<ffffffff810edec9>] deactivate_super+0x49/0x70
> >  [<ffffffff811081a4>] mntput_no_expire+0x84/0xe0
> >  [<ffffffff811083ff>] release_mounts+0x9f/0xc0
> >  [<ffffffff81108485>] put_mnt_ns+0x65/0x80
> >  [<ffffffff8122cb66>] nfs_follow_remote_path+0x1e6/0x420
> >  [<ffffffff8122cecf>] nfs4_try_mount+0x6f/0xd0
> >  [<ffffffff8122cfd2>] nfs4_get_sb+0xa2/0x360
> >  [<ffffffff810edbc8>] vfs_kern_mount+0x88/0x1f0
> >  [<ffffffff810edda2>] do_kern_mount+0x52/0x130
> >  [<ffffffff81963d6a>] ? _lock_kernel+0x6a/0x170
> >  [<ffffffff81108dae>] do_mount+0x26e/0x7f0
> >  [<ffffffff81106a4a>] ? copy_mount_options+0xea/0x190
> >  [<ffffffff811093c8>] sys_mount+0x98/0xf0
> >  [<ffffffff810024d8>] system_call_fastpath+0x16/0x1b
> > 1 lock held by mount.nfs4/3812:
> >  #0:  (&type->s_umount_key#24){+.+...}, at: [<ffffffff810edec1>] deactivate_super+0x41/0x70
> 
> I've tried both v2.6.36-rc2 and commit
> 253c34e9b10c30d3064be654b5b78fbc1a8b1896 [1] - and I can mount an NFS
> share with no problems:
> 
> sudo mount -t nfs4 sauron:/home/dedekind/ /mnt/sauron_home/
> 
> works fine. Any hints about how to reproduce this are welcome.

Huh.  The simple mount hits it every time for me.  I'll investigate some
more.

> I'll try to look at the code and figure out why this could happen.
> 
> So, does the mount at some point succeed? Or it is blocked forever? And
> sysrq-t output would be useful to look at as well.

It's blocked forever as far as I can tell.  I'll get a sysrq-t trace.

> Also, it is strange that 'sys_mount()' involves 'nfs4_kill_super()' - is
> this normal or this is an error path?

NFSv4 uses a temporary private namespace to look up the initial mount
path--see c02d7adf8c5429727a98bad1d039bccad4c61c50 and preceding commits
for explanation.  So this may well be normal (but I haven't looked at it
closely).

Hm, my mount path has a mountpoint in it--if sauron:/home/dedekind/
doesn't, then that's a difference between our setups.

> [1]: the kernel tree does not compile on this commit, and I applied
> patch on top to solve the compilation issue:
> 387ac089361fbe5ef287e6950c5c40f6b18e5c55 "block: fix missing export of
> blk_types.h"

Maybe you only hit that if you do headers_install or headers_check?

--b.