linux-kernel - Re: [PATCH 4/4] usb: libusual: locking cleanup

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Message-Id: <20071221223134.1567b755.akpm@linux-foundation.org>
Date:	Fri, 21 Dec 2007 22:31:34 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Pete Zaitcev <zaitcev@...hat.com>
Cc:	Daniel Walker <dwalker@...sta.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, linux@...mer.net,
	jonathan@...masters.org, matthias.kaehlcke@...il.com,
	kjwinchester@...il.com
Subject: Re: [PATCH 4/4] usb: libusual: locking cleanup

On Fri, 21 Dec 2007 22:24:28 -0800 Pete Zaitcev <zaitcev@...hat.com> wrote:

> On Fri, 21 Dec 2007 00:00:04 -0800, Daniel Walker <dwalker@...sta.com> wrote:
> 
> > I converted the usu_init_notify semaphore to normal mutex usage, and it 
> > should still prevent the request_module before the init routine is
> > complete. Before it acted more like a complete, now the mutex protects
> > two distinct section from running at the same time..
> 
> Let's see.
> 
> > @@ -178,10 +179,7 @@ static int usu_probe_thread(void *arg)
> >  	int rc;
> >  	unsigned long flags;
> >  
> > -	/* A completion does not work here because it's counted. */
> > -	down(&usu_init_notify);
> > -	up(&usu_init_notify);
> > -
> > +	mutex_lock(&usu_probe_mutex);
> >  	rc = request_module(bias_names[type]);
> 
> When I tried it, usb-storage would not load with unresolved symbols.
> It happens if child (usu_probe_thread) runs ahead of its parent
> (usb_usual_init -> usb_register -> usu_probe). It's entirely possible,
> depending on your scheduler.

afaict Daniel's change will fix that?

He releases usu_probe_mutex once the usb_register() has completed and this
then allows the usb_probe_thread() to start working.

I'm still wondering if that theory has been tested though.

> I hate this down-up trick too

I'd be worried if you were proud of it ;)

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