Hang due to nfs letting tasks freeze with locked inodes

[Seth Forshee <seth.forshee@canonical.com>]

We're seeing a hang when freezing a container with an nfs bind mount while
running iozone. Two iozone processes were hung with this stack trace.

 [<ffffffff81821b15>] schedule+0x35/0x80
 [<ffffffff81821dbe>] schedule_preempt_disabled+0xe/0x10
 [<ffffffff818239f9>] __mutex_lock_slowpath+0xb9/0x130
 [<ffffffff81823a8f>] mutex_lock+0x1f/0x30
 [<ffffffff8121d00b>] do_unlinkat+0x12b/0x2d0
 [<ffffffff8121dc16>] SyS_unlink+0x16/0x20
 [<ffffffff81825bf2>] entry_SYSCALL_64_fastpath+0x16/0x71

This seems to be due to another iozone thread frozen during unlink with
this stack trace:

 [<ffffffff810e9cfa>] __refrigerator+0x7a/0x140
 [<ffffffffc08e80b8>] nfs4_handle_exception+0x118/0x130 [nfsv4]
 [<ffffffffc08e9efd>] nfs4_proc_remove+0x7d/0xf0 [nfsv4]
 [<ffffffffc088a329>] nfs_unlink+0x149/0x350 [nfs]
 [<ffffffff81219bd1>] vfs_unlink+0xf1/0x1a0
 [<ffffffff8121d159>] do_unlinkat+0x279/0x2d0
 [<ffffffff8121dc16>] SyS_unlink+0x16/0x20
 [<ffffffff81825bf2>] entry_SYSCALL_64_fastpath+0x16/0x71

Since nfs is allowing the thread to be frozen with the inode locked it's
preventing other threads trying to lock the same inode from freezing. It
seems like a bad idea for nfs to be doing this.

Can nfs do something different here to prevent this? Maybe use a
non-freezable sleep and let the operation complete, or else abort the
operation and return ERESTARTSYS?

Thanks,
Seth