linux-kernel - Re: Fixing the main programmer thinko with the device model

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Message-ID: <20080324175844.GA13816@kroah.com>
Date:	Mon, 24 Mar 2008 10:58:44 -0700
From:	Greg KH <greg@...ah.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Kay Sievers <kay.sievers@...y.org>,
	"Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Fixing the main programmer thinko with the device model

On Mon, Mar 24, 2008 at 10:39:48AM -0500, James Bottomley wrote:
> Having just spent the weekend tracking two separate driver model
> problems through SCSI, I believe the biggest trap everyone falls into
> with the driver model (well, OK, at least with SCSI) is to try to defer
> a callback to the device ->release routine without realising that
> somewhere along the callback path we're going to drop a reference to the
> device.
> 
> You can do this very inadvertently:  One developer didn't realise
> bsg_unregister_queue() released a ref, and another didn't realise that
> transport_destroy_device() held one.
> 
> The real problem is that it's fantastically easy to do this ... it's not
> at all clear which of the cleanup routines actually release references
> unless you dig down into them and it's very difficult to detect because
> all that happens is that devices don't get released when they should,
> which isn't something we ever warn about.

Sounds like a documentation issue for how the scsi layer is using the
driver model more than anything else.  None of the other busses seem to
have these kinds of issues that I can see, is it just because of your
complex usage model?

> So, what I was wondering is:  is there any way we can reliably detect
> and warn when someone does this.

Warn that a device did not get released when the programmer thought it
should yet they forgot to call the correct function to have that happen?
That seems a bit difficult :)

Also note that the scsi layer usage of multiple refcounted objects
within the same structure might be causing some of these issues, that's
a bug in how the scsi layer has implmented things much more so than how
the driver core is implemented, right?

thanks,

greg k-h
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