commit 9a5d2bd99e0dfe9a31b3c160073ac445ba3d773f
Author: David Woodhouse <dwmw2@infradead.org>
Date:   Sun Apr 8 10:01:44 2012 +0000

    ppp: Fix race condition with queue start/stop
    
    Commit e675f0cc9a872fd152edc0c77acfed19bf28b81e ("ppp: Don't stop and
    restart queue on every TX packet") introduced a race condition which
    could leave the net queue stopped even when the channel is no longer
    busy. By calling netif_stop_queue() from ppp_start_xmit(), based on the
    return value from ppp_xmit_process() but *after* all the locks have been
    dropped, we could potentially do so *after* the channel has actually
    finished transmitting and attempted to re-wake the queue.
    
    Fix this by moving the netif_stop_queue() into ppp_xmit_process() under
    the xmit lock. I hadn't done this previously, because it gets called
    from other places than ppp_start_xmit(). But I now think it's the better
    option. The net queue *should* be stopped if the channel becomes
    congested due to writes from pppd, anyway.
    
    Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

commit e675f0cc9a872fd152edc0c77acfed19bf28b81e
Author: David Woodhouse <dwmw2@infradead.org>
Date:   Mon Mar 26 00:03:42 2012 +0000

    ppp: Don't stop and restart queue on every TX packet
    
    For every transmitted packet, ppp_start_xmit() will stop the netdev
    queue and then, if appropriate, restart it. This causes the TX softirq
    to run, entirely gratuitously.
    
    This is "only" a waste of CPU time in the normal case, but it's actively
    harmful when the PPP device is a TEQL slave — the wakeup will cause the
    offending device to receive the next TX packet from the TEQL queue, when
    it *should* have gone to the next slave in the list. We end up seeing
    large bursts of packets on just *one* slave device, rather than using
    the full available bandwidth over all slaves.
    
    This patch fixes the problem by *not* unconditionally stopping the queue
    in ppp_start_xmit(). It adds a return value from ppp_xmit_process()
    which indicates whether the queue should be stopped or not.
    
    It *doesn't* remove the call to netif_wake_queue() from
    ppp_xmit_process(), because other code paths (especially from
    ppp_output_wakeup()) need it there and it's messy to push it out to the
    other callers to do it based on the return value. So we leave it in
    place — it's a no-op in the case where the queue wasn't stopped, so it's
    harmless in the TX path.
    
    Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>



--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -968,7 +968,6 @@ ppp_start_xmit(struct sk_buff *skb, stru
 	proto = npindex_to_proto[npi];
 	put_unaligned_be16(proto, pp);
 
-	netif_stop_queue(dev);
 	skb_queue_tail(&ppp->file.xq, skb);
 	ppp_xmit_process(ppp);
 	return NETDEV_TX_OK;
@@ -1063,6 +1062,8 @@ ppp_xmit_process(struct ppp *ppp)
 		   code that we can accept some more. */
 		if (!ppp->xmit_pending && !skb_peek(&ppp->file.xq))
 			netif_wake_queue(ppp->dev);
+		else
+			netif_stop_queue(ppp->dev);
 	}
 	ppp_xmit_unlock(ppp);
 }
