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

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
Message-ID: <20110803135044.GM17489_at_deviant.kiev.zoral.com.ua>

> On Wed, Aug 03, 2011 at 02:44:23PM +0900, Kohji Okuno wrote:
>> Hello,
>> 
>> > Hello,
>> > 
>> > I think that devfs is sure to have the bug.
>> > I found that I couldn't open "/dev/XXX" though the kernel detected XXX
>> > device.
>> > 
>> > 
>> > "dm->dm_generation" is updated with "devfs_generation" in
>> > devfs_populate(), and the context holds only "dm->dm_lock" in
>> > devfs_populate().
>> > 
>> > On the other hand, "devfs_generation" is incremented in devfs_create()
>> > and devfs_destroy() the context holds only "devmtx" in devfs_create()
>> > and devfs_destroy().
>> > 
>> > If a context executes devfs_create() when other context is executing
>> > (***), then "dm->dm_generation" is updated incorrect value.
>> > As a result, we can not open the last detected device (we receive ENOENT).
>> > 
>> > I think that we should change the lock method.
>> > May I have advice?
>> > 
>> > void
>> > devfs_populate(struct devfs_mount *dm)
>> > {
>> > 
>> >         sx_assert(&dm->dm_lock, SX_XLOCKED);
>> >         if (dm->dm_generation == devfs_generation)
>> >                 return;
>> >         while (devfs_populate_loop(dm, 0))
>> >                 continue;
>> > (***)
>> >         dm->dm_generation = devfs_generation;
>> > }
>> > 
>> > void
>> > devfs_create(struct cdev *dev)
>> > {
>> >         struct cdev_priv *cdp;
>> > 
>> >         mtx_assert(&devmtx, MA_OWNED);
>> >         cdp = cdev2priv(dev);
>> >         cdp->cdp_flags |= CDP_ACTIVE;
>> >         cdp->cdp_inode = alloc_unrl(devfs_inos);
>> >         dev_refl(dev);
>> >         TAILQ_INSERT_TAIL(&cdevp_list, cdp, cdp_list);
>> >         devfs_generation++;
>> > }
>> > 
>> > void
>> > devfs_destroy(struct cdev *dev)
>> > {
>> >         struct cdev_priv *cdp;
>> > 
>> >         mtx_assert(&devmtx, MA_OWNED);
>> >         cdp = cdev2priv(dev);
>> >         cdp->cdp_flags &= ~CDP_ACTIVE;
>> >         devfs_generation++;
>> > }
>> > 
>> > Thanks.
>> 
>> I propose the solution. May I have advice?
>> 
>> void
>> devfs_populate(struct devfs_mount *dm)
>> {
>> 
>>         sx_assert(&dm->dm_lock, SX_XLOCKED);
>> #if 1 /* I propose... */
>>         int tmp_generation;
>> retry:
>>         tmp_generation = devfs_generation;
>>         if (dm->dm_generation == tmp_generation)
>>                 return;
>>         while (devfs_populate_loop(dm, 0))
>>                 continue;
>>         if (tmp_generation != devfs_generation)
>>                 goto retry;
>>         dm->dm_generation = tmp_generation;
>> #else /* Original */
>>         if (dm->dm_generation == devfs_generation)
>>                 return;
>>         while (devfs_populate_loop(dm, 0))
>>                 continue;
>>         dm->dm_generation = devfs_generation;
>> #endif
>> }
> 
> 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 ?

Best regards,
 Kohji Okuno