linux-kernel - Re: [PATCH] USB testing driver: convert dev->sem to mutex

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Date:	Tue, 4 Dec 2007 23:31:51 +0100
From:	Matthias Kaehlcke <matthias.kaehlcke@...il.com>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	gregkh@...e.de, linux-usb-users@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu
Subject: Re: [PATCH] USB testing driver: convert dev->sem to mutex

El Tue, Dec 04, 2007 at 11:09:02AM -0800 Daniel Walker ha dit:

> On Tue, 2007-12-04 at 17:55 +0100, Matthias Kaehlcke wrote:
> > @@ -1990,7 +1991,7 @@ static void usbtest_disconnect (struct
> > usb_interface *intf)
> >  {
> >         struct usbtest_dev      *dev = usb_get_intfdata (intf);
> >  
> > -       down (&dev->sem);
> > +       mutex_lock(&dev->lock);
> >  
> >         usb_set_intfdata (intf, NULL);
> >         dev_dbg (&intf->dev, "disconnect\n"); 
> 
> This looks like a bit of an anomaly.. The code that isn't shown above is
> the "kfree(dev)" .. So this isn't legal since it's freeing a locked
> mutex. Here's a excerpt from the comment above mutex_lock(),
> 
>  * The mutex must later on be released by the same task that
>  * acquired it. Recursive locking is not allowed. The task
>  * may not exit without first unlocking the mutex. Also, kernel
>  * memory where the mutex resides mutex must not be freed with
>  * the mutex still locked. The mutex must first be initialized
>  * (or statically defined) before it can be locked. memset()-ing
>  * the mutex to 0 is not allowed.

you're absolutely right that freeing a locked mutex isn't a good idea

thanks for pointing it out!

-- 
Matthias Kaehlcke
Linux System Developer
Barcelona

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