linux-kernel - Re: [RFD driver-core] Lifetime problems of the current driver model

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Date:	Fri, 30 Mar 2007 16:52:51 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Tejun Heo <htejun@...il.com>
Cc:	gregkh@...e.de, hugh@...itas.com, dmitry.torokhov@...il.com,
	oneukum@...e.de, maneesh@...ibm.com, rpurdie@...ys.net,
	James.Bottomley@...elEye.com, Jeff Garzik <jgarzik@...ox.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	SCSI Mailing List <linux-scsi@...r.kernel.org>
Subject: Re: [RFD driver-core] Lifetime problems of the current driver model

On Fri, 30 Mar 2007 22:58:39 +0900,
Tejun Heo <htejun@...il.com> wrote:

> It's a little bit more convoluted than that.  Module reference count of
> zero doesn't indicate that there is no one referencing the module.  It
> just means that the module can be unloaded.  ie. There still can be any
> number of kobjects with release function backed by the module but as
> long as all of them can be deleted and released by module exit function,
> the module is unloadable at that point.
> 
> IOW, module reference count does not count number of objects depending
> on the module.  It counts the number of active usages of those objects.

We must make sure that the module is never deleted while there may be
calls to ->release functions - the exit function can only return when
all ->release calls have returned. This can be guaranteed if we (1)
don't allow the module to unload if there are outstanding kobjects (we
may need a "self destruct" knob then) or (2) make sure the ->release
functions are outside of the module (see, for example,
drivers/s390/s390_rdev.c).

(Gah, that stuff is always giving me headaches. Sorry if I'm not making
sense...)
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