On Thu, Sep 06, 2012 at 01:51:18PM -0400, J. Bruce Fields wrote:
> On Thu, Sep 06, 2012 at 11:05:26AM +0800, Guo Chao wrote:
> > On Wed, Sep 05, 2012 at 04:55:15PM -0400, J. Bruce Fields wrote:
> > > From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index 1b46439..6156135 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -3658,6 +3658,7 @@ static int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry,
> > > struct inode *new_dir, struct dentry *new_dentry)
> > > {
> > > struct inode *target = new_dentry->d_inode;
> > > + struct inode *source = old_dentry->d_inode;
> > > int error;
> > >
> > > error = security_inode_rename(old_dir, old_dentry, new_dir, new_dentry);
> > > @@ -3665,8 +3666,7 @@ static int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry,
> > > return error;
> > >
> > > dget(new_dentry);
> > > - if (target)
> > > - mutex_lock(&target->i_mutex);
> > > + lock_two_nondirectories(source, target);
> > >
> > > error = -EBUSY;
> > > if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
> > > @@ -3681,8 +3681,7 @@ static int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry,
> > > if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE))
> > > d_move(old_dentry, new_dentry);
> > > out:
> > > - if (target)
> > > - mutex_unlock(&target->i_mutex);
> > > + unlock_two_nondirectories(source, target);
> > > dput(new_dentry);
> > > return error;
> > > }
> > >
> >
> > This change also fixes a race between rename and mount.
> >
> > Apparently we avoid to rename source or target if they are
> > mountpoint. But nothing prevents source being the mountpoint
> > after d_mountpoint check because we do not hold its i_mutex.
> >
> > Thus we are actually able to rename a mountpoint.
> >
> > Rename directory should also need this care.
>
> Do you have any practical way to reproduce that race?
>
> --b.
Rename a mountpoint? Of course.
One script ...
#!/bin/bash
while true
do
mount -t sysfs nodev mnt && umount mnt
done
The other ...
#!/bin/bash
while true
do
mv mnt mnt2 && mv mnt2 mnt
done
Run them simultaneously in two consoles. When mount keeps reporting
'mount point mnt does not exist', stop them, then you will see the
familiar sysfs under mnt2.
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