Re: Bug: devfs is sure to have the bug.

Hello Kostik,

> On Thu, Aug 04, 2011 at 11:41:39AM +0900, Kohji Okuno wrote:
>> Hello Kostik,
>> 
>> From: Kostik Belousov <kostikbel_at_gmail.com>
>> Subject: Re: Bug: devfs is sure to have the bug.
>> Date: Wed, 3 Aug 2011 16:50:44 +0300
>> > I think the problem you described is real, and suggested change is right.
>> > Initially, I thought that we should work with devfs_generation as with
>> > the atomic type due to unlocked access in the devfs_populate(), but then
>> > convinced myself that this is not needed.
>> > 
>> > But also, I think there is another half of the problem. Namely,
>> > devfs_lookup() calls devfs_populate_vp(), and then does lookup with the
>> > help of devfs_lookupx(). We will miss the generation update
>> > happen after the drop of the dm_lock in devfs_populate_vp() to reacquire
>> > the directory vnode lock.
>> > 
>> > I propose the change below, consisting of your fix and also retry of
>> > population and lookup in case generations do not match for ENOENT
>> >case.
>> > 
>> > diff --git a/sys/fs/devfs/devfs_devs.c b/sys/fs/devfs/devfs_devs.c
>> > index d72ada0..8ff9bc2 100644
>> > --- a/sys/fs/devfs/devfs_devs.c
>> > +++ b/sys/fs/devfs/devfs_devs.c
>> > _at__at_ -63,7 +63,7 _at__at_ static MALLOC_DEFINE(M_CDEVP, "DEVFS1", "DEVFS cdev_priv storage");
>> >  
>> >  static SYSCTL_NODE(_vfs, OID_AUTO, devfs, CTLFLAG_RW, 0, "DEVFS filesystem");
>> >  
>> > -static unsigned devfs_generation;
>> > +unsigned devfs_generation;
>> >  SYSCTL_UINT(_vfs_devfs, OID_AUTO, generation, CTLFLAG_RD,
>> >  	&devfs_generation, 0, "DEVFS generation number");
>> >  
>> > _at__at_ -630,13 +630,15 _at__at_ devfs_populate_loop(struct devfs_mount *dm, int cleanup)
>> >  void
>> >  devfs_populate(struct devfs_mount *dm)
>> >  {
>> > +	unsigned gen;
>> >  
>> >  	sx_assert(&dm->dm_lock, SX_XLOCKED);
>> > -	if (dm->dm_generation == devfs_generation)
>> > +	gen = devfs_generation;
>> > +	if (dm->dm_generation == gen)
>> >  		return;
>> >  	while (devfs_populate_loop(dm, 0))
>> >  		continue;
>> > -	dm->dm_generation = devfs_generation;
>> > +	dm->dm_generation = gen;
>> >  }
>> >  
>> >  /*
>> > diff --git a/sys/fs/devfs/devfs_int.h b/sys/fs/devfs/devfs_int.h
>> > index cdc6aba..cb01ad1 100644
>> > --- a/sys/fs/devfs/devfs_int.h
>> > +++ b/sys/fs/devfs/devfs_int.h
>> > _at__at_ -71,6 +71,8 _at__at_ struct cdev_priv {
>> >  
>> >  #define	cdev2priv(c)	member2struct(cdev_priv, cdp_c, c)
>> >  
>> > +extern unsigned devfs_generation;
>> > +
>> >  struct cdev	*devfs_alloc(int);
>> >  int	devfs_dev_exists(const char *);
>> >  void	devfs_free(struct cdev *);
>> > diff --git a/sys/fs/devfs/devfs_vnops.c b/sys/fs/devfs/devfs_vnops.c
>> > index 955bd8b..2603caa 100644
>> > --- a/sys/fs/devfs/devfs_vnops.c
>> > +++ b/sys/fs/devfs/devfs_vnops.c
>> > _at__at_ -188,7 +188,7 _at__at_ devfs_clear_cdevpriv(void)
>> >   * On success devfs_populate_vp() returns with dmp->dm_lock held.
>> >   */
>> >  static int
>> > -devfs_populate_vp(struct vnode *vp)
>> > +devfs_populate_vp(struct vnode *vp, int dm_locked)
>> >  {
>> >  	struct devfs_dirent *de;
>> >  	struct devfs_mount *dmp;
>> > _at__at_ -199,7 +199,8 _at__at_ devfs_populate_vp(struct vnode *vp)
>> >  	dmp = VFSTODEVFS(vp->v_mount);
>> >  	locked = VOP_ISLOCKED(vp);
>> >  
>> > -	sx_xlock(&dmp->dm_lock);
>> > +	if (!dm_locked)
>> > +		sx_xlock(&dmp->dm_lock);
>> >  	DEVFS_DMP_HOLD(dmp);
>> >  
>> >  	/* Can't call devfs_populate() with the vnode lock held. */
>> > _at__at_ -242,7 +243,7 _at__at_ devfs_vptocnp(struct vop_vptocnp_args *ap)
>> >  
>> >  	dmp = VFSTODEVFS(vp->v_mount);
>> >  
>> > -	error = devfs_populate_vp(vp);
>> > +	error = devfs_populate_vp(vp, 0);
>> >  	if (error != 0)
>> >  		return (error);
>> >  
>> > _at__at_ -643,7 +644,7 _at__at_ devfs_getattr(struct vop_getattr_args *ap)
>> >  	struct devfs_mount *dmp;
>> >  	struct cdev *dev;
>> >  
>> > -	error = devfs_populate_vp(vp);
>> > +	error = devfs_populate_vp(vp, 0);
>> >  	if (error != 0)
>> >  		return (error);
>> >  
>> > _at__at_ -903,7 +904,7 _at__at_ devfs_lookupx(struct vop_lookup_args *ap, int *dm_unlock)
>> >  
>> >  		if (cdev == NULL)
>> >  			sx_xlock(&dmp->dm_lock);
>> > -		else if (devfs_populate_vp(dvp) != 0) {
>> > +		else if (devfs_populate_vp(dvp, 0) != 0) {
>> >  			*dm_unlock = 0;
>> >  			sx_xlock(&dmp->dm_lock);
>> >  			if (DEVFS_DMP_DROP(dmp)) {
>> > _at__at_ -966,19 +967,30 _at__at_ devfs_lookupx(struct vop_lookup_args *ap, int *dm_unlock)
>> >  static int
>> >  devfs_lookup(struct vop_lookup_args *ap)
>> >  {
>> > -	int j;
>> >  	struct devfs_mount *dmp;
>> > -	int dm_unlock;
>> > +	int error, dm_unlock;
>> >  
>> > -	if (devfs_populate_vp(ap->a_dvp) != 0)
>> > +	dm_unlock = 0;
>> > +retry:
>> > +	if (devfs_populate_vp(ap->a_dvp, dm_unlock) != 0)
>> >  		return (ENOTDIR);
>> >  
>> >  	dmp = VFSTODEVFS(ap->a_dvp->v_mount);
>> >  	dm_unlock = 1;
>> > -	j = devfs_lookupx(ap, &dm_unlock);
>> > -	if (dm_unlock == 1)
>> > +	error = devfs_lookupx(ap, &dm_unlock);
>> > +	if (error == ENOENT) {
>> > +		if (dm_unlock)
>> > +			sx_assert(&dmp->dm_lock, SA_XLOCKED);
>> > +		else {
>> > +			sx_xlock(&dmp->dm_lock);
>> > +			dm_unlock = 1;
>> > +		}
>> > +		if (devfs_generation != dmp->dm_generation)
>> > +			goto retry;
>> > +	}
>> > +	if (dm_unlock)
>> >  		sx_xunlock(&dmp->dm_lock);
>> > -	return (j);
>> > +	return (error);
>> >  }
>> >  
>> >  static int
>> > _at__at_ -1202,7 +1214,7 _at__at_ devfs_readdir(struct vop_readdir_args *ap)
>> >  	}
>> >  
>> >  	dmp = VFSTODEVFS(ap->a_vp->v_mount);
>> > -	if (devfs_populate_vp(ap->a_vp) != 0) {
>> > +	if (devfs_populate_vp(ap->a_vp, 0) != 0) {
>> >  		if (tmp_ncookies != NULL)
>> >  			ap->a_ncookies = tmp_ncookies;
>> >  		return (EIO);
>> > _at__at_ -1565,7 +1577,7 _at__at_ devfs_symlink(struct vop_symlink_args *ap)
>> >  	if (error)
>> >  		return(error);
>> >  	dmp = VFSTODEVFS(ap->a_dvp->v_mount);
>> > -	if (devfs_populate_vp(ap->a_dvp) != 0)
>> > +	if (devfs_populate_vp(ap->a_dvp, 0) != 0)
>> >  		return (ENOENT);
>> >  
>> >  	dd = ap->a_dvp->v_data;
>> 
>> Thank you for your comment.
>> But, now I'm using 8.1-RELEASE. May I have advice about 8.X ?
> Do you mean a patch for the stable/8 ? I believe it is enough to
> apply rev. 211628 to stable/8, then the patch I posted yesterday
> should be compilable.

I'm sorry.
I need a patch for 8.1-RELEASE. Could you propose patch?

Thanks,
 Kohji Okuno