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

- From: Oliver Neukum \<oneukum@suse.com\>
- Date: 2015-07-23 09:16:02 
- List: netdev
- Page: https://ratatoskr.run/netdev/2015/07/4541335
- Thread: https://ratatoskr.run/netdev/2015/07/4539045/t.md

```text
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
```