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Subjectoopses in kobjects in 2.6.0-test11 (was Re: kobject patch)
Ok, I'm ccing lkml and everyone else who has been in on this thread at
different times. This is based on a patch from Andrey that was/is in
the -mm tree for a while.

On Mon, Dec 08, 2003 at 02:25:26PM -0800, Greg KH wrote:
> On Thu, Oct 09, 2003 at 01:48:37AM -0700, Andrew Morton wrote:
> >
> > I've had this in -mm for a while. What to do with it?
>
> Heh, nothing like digging up something from the past that I insisted was
> not needed, but...
>
> > It is possible that parent is removed before child when child is in use.
> > Trivial example is mounted USB storage when you unplug it. The kobject for
> > USB device is removed but subordinate SCSI device remains. Then kernel oopses
> > on attempt to release child e.g. umount removed USB storage. This patch fixes
> > two problems:
>
> Yes, I now think this patch needs to be applied. I can easily cause a
> parent device in sysfs to go away, with the child still present:
> - plug in a usb-serial device
> - run 'cat /dev/ttyUSB0'
> - yank the device out.
> Now if you cancel the 'cat' program, lovely oopses...
>
> So, Andrew, very sorry about this, but this patch should be sent to
> Linus. I think Pat agrees with me, but he's on the road for a few days.
> You might want to wait for him.

Hm, wait, Pat objected to the patch to kobject.c (now that I went back
and read the whole thread.) And I agree with him, but I'm now getting
an oops in get_kobj_path_length if I do the above while loading down the
machine with other tasks when I cancel 'cat'.

So something else bad is happening here...

> > - kset_hotplug. It oopses in get_kobj_path_length because child->parent
> > points to nowhere - even if parent has not yet been overwritten, its name
> > is already freed.
> >
> > The patch moves kobject_put for parent from unlink() into
> > kobject_cleanup for child making sure reference to parents exists for as
> > long as child is there and may use it.

But you can't do this, as you need that kobject_put() in unlink() for
when it is called from kobject_add().

Hm, wait... I think we are close...

Ok, here's how a parent can be removed from the system without the child
going away:
- create parent and register it successfully.
- create child, call kobject_add() which increments the count of
the parent.
- call kobject_get() on the child.
- call kobject_del() on the parent. This will keep the parent
around, as the child still has a reference on it.
- call kobject_del() on the child. This will decrement the
count on the parent due to the call in unlink(). That will
free the parent up from memory. But this child still has a
incremented count (rightly, as it is in use).

- So the child now has a stale parent pointer, causing all sorts
of fun...

I'll work on a patch for kobject.c and post it in the next message, and
include the original message and patch below for others to see.

thanks,

greg k-h

> > - after this oops has been fixed I got next one now in sysfs. The
> > problem is sysfs_remove_dir would unlink all children including
> > directories for subordinate kobjects. Resulting in dget/dput mismatch.
> > I usually got oops due to the fact that d_delete in remove_dir would free
> > inode and then simple_rmdir would try to access it.
> >
> > The patch avoids calling extra d_delete/unlink on already-deleted
> > dentry. I hate this patch but anything better apparently requires
> > complete redesign of sysfs implementation. Unlinking busy directory is
> > otherwise impossible and I am afraid it will show itself somewhere else.
> >
> >
> >
> > 25-akpm/fs/sysfs/dir.c | 12 ++++++++++--
> > 25-akpm/lib/kobject.c | 4 ++--
> > 2 files changed, 12 insertions(+), 4 deletions(-)
> >
> > diff -puN fs/sysfs/dir.c~kobject-oops-fixes fs/sysfs/dir.c
> > --- 25/fs/sysfs/dir.c~kobject-oops-fixes Thu Oct 9 01:46:51 2003
> > +++ 25-akpm/fs/sysfs/dir.c Thu Oct 9 01:46:51 2003
> > @@ -82,8 +82,16 @@ static void remove_dir(struct dentry * d
> > {
> > struct dentry * parent = dget(d->d_parent);
> > down(&parent->d_inode->i_sem);
> > - d_delete(d);
> > - simple_rmdir(parent->d_inode,d);
> > + /*
> > + * It is possible that parent has already been removed, in which
> > + * case directory is already unhashed and dput.
> > + * Note that this won't update parent->d_inode->i_nlink; OTOH
> > + * parent should already be dead
> > + */
> > + if (!d_unhashed(d)) {
> > + d_delete(d);
> > + simple_rmdir(parent->d_inode,d);
> > + }
> >
> > pr_debug(" o %s removing done (%d)\n",d->d_name.name,
> > atomic_read(&d->d_count));
> > diff -puN lib/kobject.c~kobject-oops-fixes lib/kobject.c
> > --- 25/lib/kobject.c~kobject-oops-fixes Thu Oct 9 01:46:51 2003
> > +++ 25-akpm/lib/kobject.c Thu Oct 9 01:46:51 2003
> > @@ -236,8 +236,6 @@ static void unlink(struct kobject * kobj
> > list_del_init(&kobj->entry);
> > up_write(&kobj->kset->subsys->rwsem);
> > }
> > - if (kobj->parent)
> > - kobject_put(kobj->parent);
> > kobject_put(kobj);
> > }
> >
> > @@ -457,6 +455,8 @@ void kobject_cleanup(struct kobject * ko
> > if (kobj->k_name != kobj->name)
> > kfree(kobj->k_name);
> > kobj->k_name = NULL;
> > + if (kobj->parent)
> > + kobject_put(kobj->parent);
> > if (t && t->release)
> > t->release(kobj);
> > if (s)
> >
> > _
-
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