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Date: Sat, 16 Sep 2017 00:43:43 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount
 completion
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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.