Re: 2.6.39-rc0: umount tmpfs hangs

[Al Viro]

On Mon, Mar 21, 2011 at 05:24:08AM +0300, Alexander Beregalov wrote:
> Hi
> 
> kernel is 2.6.38-06507-ga952baa
> 
> /etc/init.d/swap stop
> runs
> umount -a -t tmpfs
> 
> which is hangs at
> 
> $cat /proc/4453/stack
> [<c1066332>] rcu_barrier_sched+0x32/0x40
> [<c109fdca>] deactivate_locked_super+0x3a/0x60
> [<c10a05f6>] deactivate_super+0x46/0x60
> [<c10b6f2a>] mntput_no_expire+0x5a/0x90
> [<c10b7a6d>] sys_umount+0x5d/0x300
> [<c10b7d29>] sys_oldumount+0x19/0x20
> [<c136eb50>] sysenter_do_call+0x12/0x36
> [<ffffffff>] 0xffffffff

rcu_barrier() hanging for some reason...  I really wonder what's going
on there - missing rcu_read_unlock()?  Could you try to reproduce that
sucker and, if possible, bisect it?
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