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
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