Deadlock with umount -f involving tmpfs on top of ZFS on r271170

[Peter Holm]

On Wed, Sep 24, 2014 at 01:27:58PM +0300, Konstantin Belousov wrote:
> On Tue, Sep 23, 2014 at 08:53:19PM -0500, Bryan Drewery wrote:
> > I tried your patch and still ran into the deadlock. Here is the debug
> > information: https://people.freebsd.org/~bdrewery/vfs_deadlock.2.txt
> 
> I see what is going on.  Previous patch cannot help there.
> 
> The problem is that kern_linkat() starts write with vn_start_write(9),
> and then  tries to execute namei(9) with a path potentially crossing
> the mount point.  If the mount point is being unmounted, unmount cannot
> lay suspension on the filesystem due to writer, but it owns the covered
> vnode lock.  On the other hand, namei(9) is unable to cross the mount
> point since covered vnode is locked, thus writer does not make progress.
> 
> The similar issue exists in rename code. The deadlock is only possible
> for filesystems which suspend writes on unmount; such fs are UFS and tmpfs.
> 
> Try this.
> 
> diff --git a/sys/kern/vfs_syscalls.c b/sys/kern/vfs_syscalls.c
> index b3b7ed5..9775480 100644

I had been trying different scenarios without much luck, but with
your description I got the problem instantly. :)

The patch is an improvement, but:

http://people.freebsd.org/~pho/stress/log/kostik718.txt

- Peter