Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion
From: Jaegeuk Kim
Date: Tue Sep 19 2017 - 11:55:53 EST
On 09/16, Al Viro wrote:
> On Fri, Sep 15, 2017 at 04:29:11PM -0700, Jaegeuk Kim wrote:
> > The mntput() in delayed_fput() is the last function call. So before that moment,
> > sys_umount() may see mnt_get_count() as 2, so it avoids EBUSY condition. I'm not
> > sure why it check over 2 tho.
> Because it has just grabbed a reference itself, in addition to the one that keeps
> the damn thing alive (due to being mounted). So it bloody well should have
> triggered -EBUSY, if they refer to the same vfsmount.
I've tracked another view in terms of mnt_get_count() and sb->s_active based on
namespaces, and could get the below scenario for instance.
1. create_new_namespaces() creates ns1 and ns2,
/data(1) ns1(1) ns2(1)
| | |
sb->s_active = 3
2. after binder_proc_clear_zombies() for ns2 and ns1 triggers
sb->s_active = 2
3. umount() for /data is successed.
sb->s_active = 1
4. device_shutdown() by init
5. - delayed_mntput_work(ns1)
- put_super(), since sb->s_active = 0
Please let me know, if I'm missing something.