linux-kernel - Re: [PATCH 09/30] r/o bind mounts: drop write during emergency remount

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Date:	Sat, 23 Feb 2008 13:38:54 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	haveblue@...ibm.com, linux-kernel@...r.kernel.org, hch@....de,
	akpm@...l.org
Subject: Re: [PATCH 09/30] r/o bind mounts: drop write during emergency remount

On Mon, Feb 18, 2008 at 05:29:15PM +0100, Miklos Szeredi wrote:
> >  	file_list_lock();
> >  	list_for_each_entry(f, &sb->s_files, f_u.fu_list) {
> > -		if (S_ISREG(f->f_path.dentry->d_inode->i_mode) && file_count(f))
> > -			f->f_mode &= ~FMODE_WRITE;
> > +		struct vfsmount *mnt;
> > +		if (!S_ISREG(f->f_path.dentry->d_inode->i_mode))
> > +		       continue;
> > +		if (!file_count(f))
> > +			continue;
> > +		if (!(f->f_mode & FMODE_WRITE))
> > +			continue;
> > +		f->f_mode &= ~FMODE_WRITE;
> > +		mnt = f->f_path.mnt;
> 
> AFAICS an mntget() is needed here to make sure mnt doesn't go away
> after releasing file_list_lock.

	Good catch, but... that's not all.  We also have to move
drop_file_write_access() in __fput() past the file_kill(), or we'll
get a race here - CPU 1 enters that loop, sees ->f_count 1, checks
that file is writable, CPU 2 does final fput() and proceeds to
do drop_file_write_access() and mnt_drop_write().  CPU 1, in the
meanwhile, removes FMODE_WRITE and goes on to do mnt_drop_write()
itself.

	Oh, well...  Fixed.
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