linux-kernel - Re: [PATCH RFD] alternative kobject release wait mechanism

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Date:	Thu, 26 Apr 2007 10:58:45 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
cc:	Greg KH <greg@...ah.com>, Tejun Heo <htejun@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [PATCH RFD] alternative kobject release wait mechanism

On Thu, 26 Apr 2007, Cornelia Huck wrote:

> On Wed, 25 Apr 2007 16:13:12 -0400 (EDT),
> Alan Stern <stern@...land.harvard.edu> wrote:
> 
> > > Documentation/driver-model/lifetime-rules.txt?
> > 
> > When (if) such a file is added, it should contain more than just these few 
> > paragraphs.
> 
> This file may be a good idea in general. I'll see if I can come up with
> something useful.

Good.

> > > The remove() method must also unset driver_data.
> > 
> > It doesn't really have to.  The driver core could do it.
> 
> I think it is more consistent if the driver takes care of the fields
> specifically designed for its usage.

Yes.  However if the driver forgets to clear the field it shouldn't cause
a warning.  After all, there won't be any harm; the next driver to bind 
to the device will just overwrite the driver_data anyway.

> > If every driver follows this rule, we may not need the kobject->owner 
> > stuff.  Although it wouldn't hurt to keep it for safety's sake.
> 
> I'm still concerned about that problem. It is that there is simply no
> guarantee that everybody released their reference before the module's
> exit function returned (the driver itself can and must assure that it
> drops its last reference before it finishes exit, but it just can't
> control who else holds a reference). I'm much in favour of adding a bit
> of code than to risk oopses about which the driver can't do anything
> itself (plus, the race exists now, but the immediate disconnect will
> involve auditing all drivers).

I agree.

> > > > 		The driver must restrict itself to reading (not
> > > > 		writing!) the fields in the device structure.  The
> > > > 		only exception is that the driver may lock/unlock
> > > > 		dev->sem.
> > > 
> > > And it may decrease the reference count, of course :)
> > 
> > :-)  Actually this should be a little more general, since the device will 
> > generally be embedded in a larger structure created by the subsystem.  The 
> > driver should also be able to acquire and release locks in that larger 
> > structure.
> 
> Yes. Especially since the "gone"-field may be contained in that
> embedding structure if the subsystem controls it.

No, no!  The "gone" flag must be in the private data structure.  If it 
were in a container of the device structure, then it could be overwritten 
when a different driver binds to the device.

Alan Stern

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