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

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Date:	Wed, 27 May 2009 16:40:38 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	Hannes Reinecke <hare@...e.de>, Kay Sievers <kay.sievers@...y.org>,
	SCSI development list <linux-scsi@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	<linux-fsdevel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 25/20] sysfs: Only support removing emtpy sysfs  directories.

On Wed, 27 May 2009, James Bottomley wrote:

> On Wed, 2009-05-27 at 14:07 -0400, Alan Stern wrote:
> > On Wed, 27 May 2009, James Bottomley wrote:
> > 
> > > By refcount, I mean count of underlying devices.
> > 
> > Does that mean only registered devices, or does it include devices 
> > which are unregistered but not yet released?
> 
> All devices ... scsi_device has to has a target parent before its
> usable.

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()?

> > > Um, well, that's roughly how I said we'd have to fix all of this in the
> > > email to hannes ... it would be much easier if we could make a del'd
> > > device visible,
> > 
> > I don't follow.  Why would you want to delete a target before the host
> > is removed and then make it visible again later?  Because it doesn't
> > have any underlying devices at the moment but may gain some later on?
> 
> 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 
point there's no reason to keep an unpopulated target around, right?

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.

Alan Stern

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