Re: [PATCH 01/33] vfs: syscall: Add open_tree(2) to reference or clone a mount [ver #11]

[Al Viro]

On Thu, Aug 02, 2018 at 06:31:06PM +0100, Alan Jenkins wrote:
> Hi
> 
> I found this interesting, though I don't entirely follow the kernel
> mount/unmount code.  I had one puzzle about the code, and two questions
> which I was largely able to answer.
> 
> On 01/08/18 16:24, David Howells wrote:
> > +void dissolve_on_fput(struct vfsmount *mnt)
> > +{
> > +	namespace_lock();
> > +	lock_mount_hash();
> > +	mntget(mnt);
> > +	umount_tree(real_mount(mnt), UMOUNT_SYNC);
> > +	unlock_mount_hash();
> > +	namespace_unlock();
> > +}
> 
> Can I ask why  UMOUNT_SYNC is used here?  I feel like I must have missed
> something, but doesn't it skip the IS_MNT_LOCKED() check in
> disconnect_mount() ?
> 
> UMOUNT_SYNC seems used for non-lazy unmounts, and in internal cleanups where
> userspace wouldn't be able to see.  But I think userspace can keep watching
> in this case, e.g. by `fd2 = openat(fd, ".", O_PATH)` (or `fd2 =
> open_tree(fd, ".", 0)` ?).  I would think this function should avoid using
> UMOUNT_SYNC, like lazy unmount avoids it.

FWIW, I suspect that UMOUNT_CONNECTED might be the right thing here...