Re: umount of unreachable NFS without -f [maybe PATCH]

Stanislav Brabec
2014-09-03T13:30:16Z

Karel Zak wrote:
> On Tue, Sep 02, 2014 at 09:09:38PM +0200, Stanislav Brabec wrote:
> > I just analyzed causes of hang while calling umount without -f on an
> > unreachable NFS.
> >
> > The first hang happens on in lookup_umount_fs():
> >
> > stat(tgt, &st) == 0 && S_ISDIR(st.st_mode)
> >
> > I don't exactly know, what its removal causes, but I tried it.
> >
> > I entered into next hang in mnt_resolve_path() inside has_utab_entry().
> > We cannot prevent this hang in all cases, but preventing it in a
> > standard situation is pretty straightforward: Try to match provided
> > target name before trying to canonicalize.
> >
> > Now my umount without -f works well at least in the case, when the mount
> > point is recorded in the utab.
> >
> > If it is not recorded in utab, it still hangs later in
> > lookup_umount_fs() on:
> >
> > if (statfs(tgt, &vfs) == 0)
> >
> > Well, we can prevent even this hang, but it would be at cost of parsing
> > mtab. I read your git comments and I see, that the whole purpose of the
> > code is preventing of parsing of the generated mtab/mountinfo.
> >
> > Would be parsing of fstab a safe way without paying this cost?
>
> How often do you have NFS mounts without entry within utab? I think it's
> pretty unusual (I guess you have mount.nfs linked with libmount).

I did it somehow. Guessing that it could have something with my
experiments, killing hanging umount with signal 11 etc.

> The change in has_utab_entry() is definitely good idea, but I'm not
> sure about the rest. Maybe it would be enough to call has_utab_entry()
> before
>
> stat(tgt, &st) == 0 && S_ISDIR(st.st_mode)
>
>
> then it will be good enough for almost all NFS mounts (with utab
> entries).

Yes, I just verified that it helps as well.

But I found just another stat() call further in
mnt_context_find_umount_fs():
if (stat(tgt, &st) == 0 && S_ISDIR(st.st_mode)) {
before cache lookups.

And next there is:
cn_tgt = mnt_resolve_path(tgt, cache);
which will probably hang as well.

I can imagine that we will not uses stat() here, but I cannot imagine,
how to set filter correctly without canonicalization.

I am just curious why one of my umount attempts of unresponsive NFS
succeeded yesterday...

--
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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