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

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Message-Id: <20071221222428.a75a5a34.zaitcev@redhat.com>
Date:	Fri, 21 Dec 2007 22:24:28 -0800
From:	Pete Zaitcev <zaitcev@...hat.com>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	akpm@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, linux@...mer.net,
	jonathan@...masters.org, matthias.kaehlcke@...il.com,
	kjwinchester@...il.com, zaitcev@...hat.com
Subject: Re: [PATCH 4/4] usb: libusual: locking cleanup

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.

I hate this down-up trick too, so if you have a better idea, I'm all ears.

-- Pete
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