Re: 6.0 hangs (while building OOo)

On Friday 14 October 2005 01:56 pm, Don Lewis wrote:
> On 13 Oct, Don Lewis wrote:
> > The bug is that once we have unlocked pdp, another thread can do a
> > lookup and overwrite dp->i_ino, so instead of getting the vnode for the
> > ".." directory entry, VFS_VGET() will return the vnode for a
> > subdirectory of the current directory, and when we relock the current
> > directory we'll have a lock order reversal.
> >
> > Even if this doesn't result in a deadlock, it looks like it has the
> > potential for mucking up lookups that involve "..".  I also don't
> > currently see a way for this to become a vnode lock leak.
>
> I think the leak happens when dp->i_ino gets overwritten by the inode
> number for ".".  This causes ufs_lookup() to recurse on the lock for the
> current directory vnode (the lock is first acquired by VFS_VGET() and
> then recursed by vn_lock()). This isn't expected by lookup(), which
> compares whether the vnode returned by VOP_LOOKUP() is the same as the
> directory vnode and uses this information to decide whether to call
> vput() or vrele().
>
> > The fix is to preserve a copy of dp->d_ino before unlocking pdp,
> > and pass the saved value to VFS_VGET().
> >
> > Index: sys/ufs/ufs/ufs_lookup.c
> > ===================================================================
> > RCS file: /home/ncvs/src/sys/ufs/ufs/ufs_lookup.c,v
> > retrieving revision 1.77
> > diff -u -r1.77 ufs_lookup.c
> > --- sys/ufs/ufs/ufs_lookup.c	13 Apr 2005 10:59:09 -0000	1.77
> > +++ sys/ufs/ufs/ufs_lookup.c	13 Oct 2005 23:20:59 -0000
> > _at__at_ -153,6 +153,7 _at__at_
> >  	int flags = cnp->cn_flags;
> >  	int nameiop = cnp->cn_nameiop;
> >  	struct thread *td = cnp->cn_thread;
> > +	u_int32_t saved_ino;
> >
> >  	bp = NULL;
> >  	slotoffset = -1;
> > _at__at_ -557,8 +558,9 _at__at_
> >  	 */
> >  	pdp = vdp;
> >  	if (flags & ISDOTDOT) {
> > +		saved_ino = dp->i_ino;
> >  		VOP_UNLOCK(pdp, 0, td);	/* race to get the inode */
> > -		error = VFS_VGET(pdp->v_mount, dp->i_ino,
> > +		error = VFS_VGET(pdp->v_mount, saved_ino,
> >  		    cnp->cn_lkflags, &tdp);
> >  		vn_lock(pdp, LK_EXCLUSIVE | LK_RETRY, td);
> >  		if (error)

Sounds good to me.  Good sleuthing!

-- 
John Baldwin <jhb_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org