From: Eugene Shatokhin <eugene.shatokhin@rosalab.ru>
To: Oliver Neukum <oneukum@suse.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
linux-usb@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: Several races in "usbnet" module (kernel 4.1.x)
Date: Fri, 24 Jul 2015 17:41:58 +0300 [thread overview]
Message-ID: <55B24EB6.5090907@rosalab.ru> (raw)
In-Reply-To: <1437642952.4377.10.camel@suse.com>
23.07.2015 12:15, Oliver Neukum пишет:
> 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
From what I see now in Documentation/atomic_ops.txt, stores to the
properly aligned memory locations are in fact atomic.
So, I think, the situation you described above cannot happen for
dev->flags, which is good. No need to address that in the patch. The
race might be harmless after all.
If I understand the code correctly now, dev->flags is set to 0 in
usbnet_stop() so that the worker function (usbnet_deferred_kevent) would
do nothing, should it start later. If so, how about adding memory
barriers for all CPUs to see dev->flags is 0 before other things?
The patch could look like this then:
--------------------
diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 3c86b10..d87b9c7 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;
+ smp_mb(); /* make sure the workers see that dev->flags == 0 */
+
del_timer_sync (&dev->delay);
tasklet_kill (&dev->bh);
+
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);
@@ -1078,6 +1081,9 @@ usbnet_deferred_kevent (struct work_struct *work)
container_of(work, struct usbnet, kevent);
int status;
+ /* See the changes in dev->flags from other CPUs. */
+ smp_mb();
+
/* usb_clear_halt() needs a thread context */
if (test_bit (EVENT_TX_HALT, &dev->flags)) {
unlink_urbs (dev, &dev->txq);
--------------------
What do you think?
Regards,
Eugene
next prev parent reply other threads:[~2015-07-24 14:41 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-20 18:13 Several races in "usbnet" module (kernel 4.1.x) Eugene Shatokhin
2015-07-21 12:04 ` Oliver Neukum
[not found] ` <1437480243.3823.5.camel-IBi9RG/b67k@public.gmane.org>
2015-07-24 17:38 ` Eugene Shatokhin
2015-07-27 12:29 ` Oliver Neukum
2015-07-27 13:53 ` Eugene Shatokhin
2015-07-21 13:07 ` Oliver Neukum
[not found] ` <55AD3A41.2040100-irhHPgl+04UvJsYlp49lxw@public.gmane.org>
2015-07-21 14:22 ` Oliver Neukum
2015-07-22 18:33 ` Eugene Shatokhin
2015-07-23 9:15 ` Oliver Neukum
2015-07-24 14:41 ` Eugene Shatokhin [this message]
2015-07-27 10:00 ` Oliver Neukum
2015-07-27 14:23 ` Eugene Shatokhin
2015-08-14 16:55 ` Eugene Shatokhin
2015-08-14 16:58 ` [PATCH] usbnet: Fix two races between usbnet_stop() and the BH Eugene Shatokhin
2015-08-19 1:54 ` David Miller
[not found] ` <20150818.185407.1667358232705414236.davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>
2015-08-19 7:57 ` Eugene Shatokhin
2015-08-19 10:54 ` Bjørn Mork
2015-08-19 11:59 ` Eugene Shatokhin
2015-08-19 12:31 ` Bjørn Mork
2015-08-24 12:20 ` Eugene Shatokhin
2015-08-24 13:29 ` Bjørn Mork
2015-08-24 17:00 ` Eugene Shatokhin
2015-08-25 12:31 ` Oliver Neukum
2015-08-24 17:43 ` David Miller
[not found] ` <20150824.104328.554582952440857559.davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>
2015-08-24 18:06 ` Alan Stern
2015-08-24 18:21 ` Alan Stern
2015-08-25 12:36 ` Oliver Neukum
2015-08-24 18:35 ` David Miller
2015-08-24 18:12 ` Eugene Shatokhin
2015-07-23 9:43 ` Several races in "usbnet" module (kernel 4.1.x) Oliver Neukum
2015-07-23 11:39 ` Eugene Shatokhin
2015-08-24 20:13 ` [PATCH 0/2] usbnet: Fix 2 problems in usbnet_stop() Eugene Shatokhin
2015-08-24 20:13 ` [PATCH 1/2] usbnet: Get EVENT_NO_RUNTIME_PM bit before it is cleared Eugene Shatokhin
2015-08-25 13:01 ` Oliver Neukum
[not found] ` <1440507709.13824.6.camel-IBi9RG/b67k@public.gmane.org>
2015-08-25 14:16 ` Bjørn Mork
2015-08-25 14:22 ` Oliver Neukum
2015-08-26 2:44 ` David Miller
2015-08-24 20:13 ` [PATCH 2/2] usbnet: Fix a race between usbnet_stop() and the BH Eugene Shatokhin
2015-08-24 21:01 ` Bjørn Mork
2015-08-28 8:09 ` Eugene Shatokhin
2015-08-28 8:55 ` Bjørn Mork
2015-08-28 10:42 ` Eugene Shatokhin
2015-08-31 7:32 ` Bjørn Mork
2015-08-31 8:50 ` Eugene Shatokhin
2015-09-01 7:58 ` Oliver Neukum
2015-09-01 13:54 ` Eugene Shatokhin
2015-09-01 14:05 ` [PATCH] " Eugene Shatokhin
2015-09-08 7:24 ` Eugene Shatokhin
2015-09-08 7:37 ` Bjørn Mork
2015-09-08 7:48 ` Oliver Neukum
2015-09-08 20:18 ` David Miller
2015-09-01 7:57 ` [PATCH 2/2] " Oliver Neukum
2015-08-26 2:45 ` David Miller
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