Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount completion

[Jaegeuk Kim]

On 09/21, Al Viro wrote:
> On Wed, Sep 20, 2017 at 05:34:09PM -0700, Jaegeuk Kim wrote:
> > > 	flush_delayed_fput()
> > > 		does nothing, the list is empty
> > 
> > 		how about waiting for workqueue completion here?
> > 
> > > 	....
> > 
> > 	If all the __fput()s are not finished, do_umount() will return -EBUSY.
> 
> Hell, no.  That's only when they are all on the same vfsmount.  And in that
> case you don't need any waiting - if any of those mntput() is not past the
> unlock_mount_hash() in mntput_no_expire(), you will get -EBUSY.  And if they
> all are, the caller of umount(2) will end up dropping the last reference.  
> In which case the shutdown will be scheduled via task_work_add() and processed
> before umount(2) returns to userland.

Yes, what I'm trying to do with this flag would be waiting for releasing
mnt_count in the same vfsmount as well as sb->s_active across namespaces.
So, here at first, I wanted to wait for delayed_fput which grabs mnt_count
in the same vfsmount, so that do_umount() could be succeeded in time. If
this is the last remaining namespace, waiting for delayed_mntput enables
us to shut the filesystem down by task_work at the end of umount(2).

> The whole problem is that you have several vfsmounts over the same filesystem
> (== same struct super_block), some of them held by kernel threads of yours.
> umount(2) doesn't affect those and isn't affected by those.  What you do is,
> AFAICS,
> 	ask the kernel threads to start shutting down
> 	umount()
> 	shut device down, hoping that all vfsmounts that used
> to be held by those threads are gone by that point.

Yes, and actually, android retries umount(2) for several seconds, if it gets
failure. So, first I thought it'd be better to make umount() more deterministic.

> Your patch tries to stick "flush the pending work" in the umount().
> With no warranty that it will catch that stuff in the stage where
> flushing will affect anything.
> 
> > +void flush_delayed_fput_wait(void)
> > +{
> > +	delayed_fput(NULL);
> > +	flush_delayed_work(&delayed_fput_work);
> > +}
> 
> > +void flush_delayed_mntput_wait(void)
> > +{
> > +	delayed_mntput(NULL);
> > +	flush_delayed_work(&delayed_mntput_work);
> > +}
> 
> It's still a broken approach.  What I don't understand is why bother
> with that sort of brittle logics in the first place.  Why not simply
> open the damn thing with O_EXCL before proceeding to device shutdown?
> And if you get "busy" from that, wait and retry...

I'm not sure how many times we can retry and wait for this. IMHO, it'd be better
to use this together with the new flag, since this can detect unclosed namespace
given successful umount(2).