linux-kernel - [PATCH/RFA] sch_generic: fix carrier-on bug?

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Message-ID: <20071016215555.GA4732@havoc.gtf.org>
Date:	Tue, 16 Oct 2007 17:55:55 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH/RFA] sch_generic: fix carrier-on bug?

While looking at a net driver with the following construct,

	if (!netif_carrier_ok(dev))
		netif_carrier_on(dev);

it stuck me that the netif_carrier_ok() check was redundant, since
netif_carrier_on() checks bit __LINK_STATE_NOCARRIER anyway.  This is
the same reason why netif_queue_stopped() need not be called prior to
netif_wake_queue().

This is true, but there is however an unwanted side effect from assuming
that netif_carrier_on() can be called multiple times:  it touches the
watchdog, regardless of pre-existing carrier state.

The fix:  move watchdog-up inside the bit-cleared code path.

Signed-off-by: Jeff Garzik <jgarzik@...hat.com>
---

This looks like an obvious bug fix to me, but I found it by reading code
rather than hitting it in the field.  Review appreciated.

 net/sched/sch_generic.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 95ae119..e01d576 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -249,10 +249,11 @@ static void dev_watchdog_down(struct net_device *dev)
  */
 void netif_carrier_on(struct net_device *dev)
 {
-	if (test_and_clear_bit(__LINK_STATE_NOCARRIER, &dev->state))
+	if (test_and_clear_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
 		linkwatch_fire_event(dev);
-	if (netif_running(dev))
-		__netdev_watchdog_up(dev);
+		if (netif_running(dev))
+			__netdev_watchdog_up(dev);
+	}
 }

 /**
-
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