Need to force sync(2) before umounting UFS1 filesystems?

[Kirk McKusick]

> Date: Mon, 10 Oct 2011 19:12:59 -0700
> From: Garrett Cooper <yanegomi at gmail.com>
> To: Kostik Belousov <kostikbel at gmail.com>
> Cc: Kirk McKusick <mckusick at mckusick.com>, Attilio Rao <attilio at freebsd.org>,
>         Xin LI <delphij at freebsd.org>, freebsd-fs at freebsd.org
> Subject: Re: Need to force sync(2) before umounting UFS1 filesystems?
> 
> 2011/10/10 Kostik Belousov <kostikbel at gmail.com>:
> 
> > The real case to test is the NFS mount which is wedged due to
> > hung/unresponsive NFS server. I have high suspect that the patch
> > could introduce the unkillable hung unmount process.
> 
>     It blocked, but I could ^C it perfectly fine. I tested it via:
> 
> Setup:
> 1. Started up FreeNAS 8.x image; it acquired an IP from my server with
> dhcp-75.local.
> 
> Test 1:
> 1. mount -t nfs dhcp-75:/mnt/tank /mnt/nfs/ from my test workstation.
> 2. Paused VM.
> 3. umount /mnt/nfs (the command blocked).
> 4. ^C.
> 5. mount | grep /mnt/nfs showed nothing (it had unmounted).
> 
> Test 2:
> 1. mount -t nfs dhcp-75:/mnt/tank /mnt/nfs/ from my test workstation (blocked).
> 2. Opened up another ssh session and cd'ed to /mnt/nfs .
> 3. Paused VM.
> 4. umount /mnt/nfs . It failed with EBUSY.
> 5. mount | grep /mnt/nfs showed that it was still mounted, as expected.
> 
>     So unless there are buffers still waiting to be written out to an
> NFS share, or other reasons that would prevent the NFS share from
> being fully released, I doubt the proposed behavior is really
> different from previous versions of FreeBSD.
> Thanks,
> -Garrett

Given the testing that has been done and our discussion about deadlocks,
I believe that I should proceed to check in my originally proposed change.
Notably the one that simply deleted the != MNT_FORCE conditional. However,
there is no harm in using my revised version that releases the covered vnode before draining vfs_busy, and there might be some future case where that would be a necessary thing to do. 

Speak up if you think I should not proceed to check in this change.
Also, let me know if you have thoughts on which version I should use.

        Kirk McKusick