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

[Al Viro]

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.

What _is_ the semantics of UMOUNT_WAIT?  What does it guarantee,
and what would be supplying it to umount(2)?