Re: Why do very few filesystems have umount helpers

[Steve French]

On Mon, Jul 29, 2024 at 4:50 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
On Mon, Jul 29, 2024 at 12:33 PM Steve French <smfrench@xxxxxxxxx> wrote:
> > The first step should be to identify what exactly keeps your mount busy
> > in generic/044 and generic/043.
>
> That is a little tricky to debug (AFAIK no easy way to tell exactly which
> reference is preventing the VFS from proceeding with the umount and
> calling kill_sb).  My best guess is something related to deferred close
> (cached network file handles) that had a brief refcount on
> something being checked by umount, but when I experimented with
> deferred close settings that did not seem to affect the problem so
> looking for other possible causes.
>
> I just did a quick experiment by adding a 1 second wait inside umount
> and confirmed that that does fix it for those two tests when mounted to Samba,
> but not clear why the slight delay in umount helps as there is no pending
> network traffic at that point.

I did some more experimentation and it looks like the umount problem
with those two xfstests to Samba is related to IOC_SHUTDOWN.
If I return EOPNOTSUPP on IOC_SHUTDOWN
then the 1 second delay in umount is not necessary - so something that
happens after IOC_SHUTDOWN races with umount (thus the 1 second delay
that I tried as a quick experiment fixes it indirectly) in this
testcase (although
apparently this race between IOC_SHUTDOWN and umount is not an issue
to some other servers but is reproducible to Samba and ksmbd (at least
in some easy to setup configurations)



-- 
Thanks,

Steve