Re: [RFC 02/22] configfs: Add struct configfs_item_operations->check_link() in configfs_unlink()

Nicholas A. Bellinger
2010-09-10T19:56:10Z

On Fri, 2010-09-10 at 12:44 -0700, Joel Becker wrote:
> On Fri, Sep 10, 2010 at 12:06:46PM -0700, Nicholas A. Bellinger wrote:
> > On Fri, 2010-09-10 at 08:28 -0700, Joel Becker wrote:
> > > The trivial solution is to refcount your ACLs. You get both
> > > allow_link() calls, so you should be able to increment a counter there,
> > > and then drop them when the last drop_link() call is made. That will
> > > keep your consumer structures around until all links are exhausted.
> > >
> >
> > So I am a bit confused wrt to this last response.. The ->check_link()
> > patch and it's use in the fabric independent code within
> > target_core_fabric_configfs.c does exactly this for the 'MappedLUN'
> > symlink case, eg: requires the consumer to do the allow_link() +
> > drop_link() refcounting, and add the
> > API check into fs/configfs/symlink.c:configfs_unlink()
>
> You can refcount without check_link().

So what do you recommend here..?

>
> > Is there another form of configfs consumer refcounting that you had in
> > mind beyond using an atomic_t for this with ->check_link() here..?
>
> I'm saying that you won't crash if you don't free the ACLs on
> the first drop_link(). That is, the drop_link() goes through as
> configfs wants it to, but you don't crash.

The problem is that the 'unlink sub_child/group1/src_0/src_link' can't
signal to the other struct config_group to also call an internal 'unlink
sub_child/group2/dst_0/dst_link' to drop the child link outside of it's
struct config_group.

>
> > So beyond a configfs consumer solution, what do you think about checking
> > for the sub_child/group2/dst_0/dst_link style of symlink
> > in fs/configfs/symlink.c:configfs_symlink() in order to add some form of
> > internal refcount when the symlink source is within the same consumer
> > LKM, but outside of the parent struct config_group..?
> >
> > This would involve the conversion of fs/configfs/symlink.c:
> > configfs_unlink() path to check for the existence of this internal
> > refcount and returning -EPERM when any sub_child/group2/dst_0/dst_link
> > exist when 'unlink sub_child/group1/src_0/src_link' is attempted.
>
> You're still fighting allowing the links to go away. You
> haven't explained why that is necessary. You had a problem with a crash
> because you expected one reference to your ACLs and actually have two,
> but you can fix that without modifying configfs.

If this is the case then I must be mis-understanding what you mean by
configfs consumer refcounting from allow_link() and drop_link(). Can
you give me a bit more detail where I should be looking..?

Thanks!

--nab