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

From: Eugene Shatokhin <[email-masked]>
Date: Fri, 14 Aug 2015 19:55:26 +0300

Hi,

21.07.2015 17:22, Oliver Neukum пишет:
> On Mon, 2015-07-20 at 21:13 +0300, Eugene Shatokhin wrote:
>> And here, the code clears EVENT_RX_KILL bit in dev->flags, which may
>> execute concurrently with the above operation:
>> #0 clear_bit (bitops.h:113, inlined)
>> #1 usbnet_bh (usbnet.c:1475)
>>          /* restart RX again after disabling due to high error rate */
>>          clear_bit(EVENT_RX_KILL, &dev->flags);
>>
>> If clear_bit() is atomic w.r.t. setting dev->flags to 0, this race is
>> not a problem, I guess. Otherwise, it may be.
>
> clear_bit is atomic with respect to other atomic operations.
> So how about this:
>
> 	Regards
> 		Oliver
>
>>>From 1c4e685b3a9c183e04c46b661830e5c7ed35b513 Mon Sep 17 00:00:00 2001
> From: Oliver Neukum <[email-masked]>
> Date: Tue, 21 Jul 2015 16:19:40 +0200
> Subject: [PATCH] usbnet: fix race between usbnet_stop() and the BH
>
> Does this do the job?
>
> Signed-off-by: Oliver Neukum <[email-masked]>
> ---
>   drivers/net/usb/usbnet.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index 3c86b10..77a9a86 100644
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -778,7 +778,7 @@ int usbnet_stop (struct net_device *net)
>   {
>   	struct usbnet		*dev = netdev_priv(net);
>   	struct driver_info	*info = dev->driver_info;
> -	int			retval, pm;
> +	int			retval, pm, mpn;
>
>   	clear_bit(EVENT_DEV_OPEN, &dev->flags);
>   	netif_stop_queue (net);
> @@ -813,14 +813,17 @@ int usbnet_stop (struct net_device *net)
>   	 * can't flush_scheduled_work() until we drop rtnl (later),
>   	 * else workers could deadlock; so make workers a NOP.
>   	 */
> +	mpn = !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags);
>   	dev->flags = 0;
>   	del_timer_sync (&dev->delay);
>   	tasklet_kill (&dev->bh);
> +	mpn |= !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags);
> +	/* in case the bh reset a flag */
> +	dev->flags = 0;
>   	if (!pm)
>   		usb_autopm_put_interface(dev->intf);
>
> -	if (info->manage_power &&
> -	    !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags))
> +	if (info->manage_power && mpn)
>   		info->manage_power(dev, 0);
>   	else
>   		usb_autopm_put_interface(dev->intf);
>

 From what we have discussed here, I have combined a patch that fixes 
the race #1 in usbnet_stop() and makes #4 harmless by using atomics. I 
will send it shortly.

I had to make some adjustments (e.g. using spin_lock_nested in one place 
for lockdep to see it is OK to take dev->done.lock there).

I have tested the patch on the mainline kernel 4.2-rc6 built for x86-64, 
with the same USB modem. So far, lockdep, Kmemleak (just in case) and my 
tools have not detected problems in the relevant parts of the code. The 
device and the driver seem to work well.

So, what is your opinion?

Regards,
Eugene