Subject: Re: Several races in "usbnet" module (kernel 4.1.x)

From: Oliver Neukum <[email-masked]>
Date: Thu, 23 Jul 2015 11:15:52 +0200

On Wed, 2015-07-22 at 21:33 +0300, Eugene Shatokhin wrote:
> The following part is not necessary, I think. usbnet_bh() does not
> touch 
> EVENT_NO_RUNTIME_PM bit explicitly and these bit operations are
> atomic 
> w.r.t. each other.
> 
> > +     mpn |= !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags);
> > +     /* in case the bh reset a flag */

Yes, they are atomic w.r.t. each other. And that limitation worries me.

I am considering architectures which do atomic operations with
spinlocks. And this code mixes another operation into it. Can
this happen?

CPU A				CPU B

take lock
read old value
				set value to 0
clear bit
write back changed value
release lock

	Regards
		Oliver