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Date: Tue, 19 Sep 2017 08:55:47 -0700 From: Jaegeuk Kim <jaegeuk@...nel.org> To: Al Viro <viro@...IV.linux.org.uk> Cc: Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net Subject: Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion 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. Term: namespace(mnt_get_count()) 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 - delayed_fput() - delayed_mntput_work(ns2) /data(1) ns1(1) | | ---------- | sb->s_active = 2 3. umount() for /data is successed. ns1(1) | sb->s_active = 1 4. device_shutdown() by init 5. - delayed_mntput_work(ns1) - put_super(), since sb->s_active = 0 - -EIO Please let me know, if I'm missing something. Thanks,
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