From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ftp.linux.org.uk>
Subject: Re: [RFC][PATCH 03/20] Add vfsmount writer count
Date: Sun, 18 Jun 2006 19:33:20 +0100
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	herbert@13thfloor.at
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To: Dave Hansen <haveblue@us.ibm.com>
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On Fri, Jun 16, 2006 at 04:12:15PM -0700, Dave Hansen wrote:
> +/*
> + * Must be called under write lock on mnt->mnt_sb->s_umount,
> + * this prevents concurrent decrements which could make the
> + * value -1, and test in mnt_want_write() wrongly succeed
> + */
> +static inline int mnt_make_readonly(struct vfsmount *mnt)
> +{
> +	if (!atomic_dec_and_test(&mnt->mnt_writers)) {
> +		atomic_inc(&mnt->mnt_writers);
> +		return -EBUSY;
> +	}
> +	return 0;
> +}

Check in faccessat() could get screwed by that, if you just lose the
race with final writer going away.  Then mnt_make_readonly() will 
fail (as it should) and access(2) give -EROFS.