Re: [PATCH 25/20] sysfs: Only support removing emtpy sysfs directories.

Alan Stern
2009-05-27T21:31:59Z

On Wed, 27 May 2009, James Bottomley wrote:

> > I can't tell whether you understood my point. After a scsi_device is
> > unregistered but before it is released -- i.e., when its state is
> > SDEV_DEL -- it _is_ essentially unusable. So why wait until it is
> > released to decrement the target's device counter? Why not do the
> > decrement in __scsi_remove_device()?
>
> because the use model of the device still requires a valid target. Even
> though it gets gated in several places in SDEV_DEL, we still have use of
> the target parent. This is fixable, but only by a long audit of all the
> sdev uses plus the enforcement of no use of target in DEL state rule,
> which adds complexity.

You're failing to distinguish properly between "delete" and "release".
A target (or device in general) is deleted when it is removed from
visibility -- i.e., when device_del() is called. It is released when
the final put_device() call occurs and the data structure is
deallocated.

So, all I'm saying is there's nothing wrong with deleting a target
when all its children are deleted, provided the target isn't released
until all the children are released. Below you say the same thing.

> > > Perhaps I haven't made the problem clear enough. You only want early
> > > del if the host is going away, otherwise the target might be reused and
> > > it can't be if you've called del on it. So there needs to be an
> > > integration into the host lifecycle in some form.
> >
> > Yes, granted. That integration doesn't have to be complicated.
> > Basically, you just decrement the counters in all the targets when
> > setting a host's state to SHOST_DEL or SHOST_DEL_RECOVERY. At that
>
> And SHOST_CANCEL and SHOST_CANCEL_RECOVERY.

If you prefer. I thought SHOST_DEL would be more appropriate because
it occurs after scsi_forget_host() is called. All those transitions
occur in scsi_remove_host(), anyway.

> > point there's no reason to keep an unpopulated target around, right?
>
> If the child list were empty, sure. However, it's likely not going to
> be at this point.

Regardless, it will work either way.

> > Up until that point, the counter's value should be one more than the
> > number of underlying sdevs. So if the counter decrements to 0 then
> > there were no underlying sdevs and the target is deleted immediately;
> > otherwise it is deleted when the last remaining sdev is deleted.
>
> No, that's the problem. It can be removed from visibility if it has no
> visible sdevs, but it can't be deleted until the last sdev is released.

Allow me to rephrase this: A target can be removed from visibility if
it has no visible sdevs, but it can't be _released_ until the last sdev
is released.

That's fine. You remove a target from visibility when target->reap_ref
becomes 0. The target isn't released until the target's embedded
struct device's refcount becomes 0. To make this work, simply have
scsi_alloc_sdev() call

get_device(&starget->dev);

and have scsi_device_dev_release_usercontext() call

put_device(&starget->dev);

Doesn't that do exactly what you're asking for?

Alan Stern