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

[Al Viro]

On Wed, Sep 13, 2017 at 04:31:16PM -0700, Jaegeuk Kim wrote:
> Hi Al,
> 
> On 09/14, Al Viro wrote:
> > On Wed, Sep 13, 2017 at 01:09:41PM -0700, Jaegeuk Kim wrote:
> > > +	if (!retval && (flags & UMOUNT_WAIT)) {
> > > +		if (likely(!(current->flags & PF_KTHREAD)))
> > > +			task_work_run();
> > 
> > This is complete crap.  The same damn thing will be done by
> > caller of sys_umount() pretty much immediately afterwards.
> > I'm not sure what it is that you are trying to paper over,
> > but this is just plain wrong.
> 
> Okay.
> 
> > What _is_ the semantics of UMOUNT_WAIT?  What does it guarantee,
> > and what would be supplying it to umount(2)?
> 
> When android tries to reboot the system, it calls umount(2) without any flag.
> Then, mntput_no_expire() will add delayed_mntput_work() which finally does
> cleanup_mnt() later. In the mean time, android proceeded to shutdown all
> the UFS devices.

Why has task_work_add() failed?  Or is that umount(2) issued by a kernel thread?