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Fw: [BUG/PATCH] CONFIG_NET_HW_FLOWCONTROL and SMP

To: Andrew Morton <akpm@xxxxxxxx>
Subject: Fw: [BUG/PATCH] CONFIG_NET_HW_FLOWCONTROL and SMP
From: Robert Olsson <Robert.Olsson@xxxxxxxxxxx>
Date: Mon, 29 Sep 2003 23:25:24 +0200
Cc: netdev@xxxxxxxxxxx, dfages@xxxxxxxxxx
In-reply-to: <20030929123734.5bd97a47.akpm@osdl.org>
References: <20030929123734.5bd97a47.akpm@osdl.org>
Sender: netdev-bounce@xxxxxxxxxxx
Hello!

IMO HW_FLOWCONTROL is now outdated. It has served very well and encouraged 
people to use for Linux production networking. With a 2 tulip NIC's on a UP 
system as a typical system. Only tulip has code for it. This option was 
added by Alexey in 2.1.63 something...


Cheers.
                                                --ro


Andrew Morton writes:
 > 
 > 
 > Begin forwarded message:
 > 
 > Date: Mon, 29 Sep 2003 14:08:54 +0200
 > From: dfages@xxxxxxxxxx
 > To: linux-kernel@xxxxxxxxxxxxxxx
 > Subject: [BUG/PATCH] CONFIG_NET_HW_FLOWCONTROL and SMP
 > 
 > 
 > 
 > 
 > Hi,
 > by testing the CONFIG_NET_HW_FLOWCONTROL (NIC Hardware throttling) on
 > a SMP system, I found a bug in net/core/dev.c : the netdev_dropping
 > variable can be set to a negative value (the result is that a CPU can
 > be locked in "throttle" state).
 > This bug seems to exist in 2.4 and in 2.6 kernels.
 > 
 > Here's a typical scenario :
 > 
 > - Throttling --> (CPU 0)queue->throttle==1 && (CPU 1)queue->throttle==1
 >      && netdev_dropping == 2
 > - 1st packet Unthrottle ( in net_rx_action() ), received by CPU 0
 >      --> (CPU 0)queue->throttle==1 && (CPU 1)queue->throttle==1
 >      && netdev_dropping == 1
 > - 2nd packet Unthrottle ( in net_rx_action() ), received by CPU 0
 >      --> (CPU 0)queue->throttle==0 && (CPU 1)queue->throttle==1
 >      && netdev_dropping == 0
 > - 3nd packet Unthrottle ( in net_rx_action() ), received by CPU 1
 >      --> (CPU 0)queue->throttle==0 && (CPU 1)queue->throttle==0
 >      && netdev_dropping == -1
 > 
 > and so on...
 > 
 > The problem is that the (CPU)queue->throttle should be set to zero every
 > time the netdev_dropping variable is decremented.
 > 
 > 
 > Here's a patch for the 2.4.19 kernel (tested with success) :
 > 
 > --- linux-2.4.19/net/core/dev.c.orig     Mon Sep 29 12:49:14 2003
 > +++ linux-2.4.19/net/core/dev.c    Tue Sep 23 18:35:35 2003
 > @@ -1519,8 +1519,8 @@
 > 
 >  #ifdef CONFIG_NET_HW_FLOWCONTROL
 >      if (queue->throttle && queue->input_pkt_queue.qlen < no_cong_thresh ) {
 > +         queue->throttle = 0;
 >           if (atomic_dec_and_test(&netdev_dropping)) {
 > -              queue->throttle = 0;
 >                netdev_wakeup();
 >                goto softnet_break;
 >           }
 > 
 > 
 > 
 > 
 > I haven't done a patch for 2.4.22 (as we currently use 2.4.19) but the same
 > modification should be applied around lines 1572 to 1577 of net/core/dev.c
 > 
 > For 2.6.0-test5, the lines to modified are arount 1631 to 1637.
 > 
 > Regards,
 > ---
 > Daniel FAGES
 > ARKOON Network Security    http://www.arkoon.net
 > 
 > 
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