RE: [PATCH -rt] Preemption problem in kernel RT Patch

["Beauchemin, Mark" <Mark.Beauchemin@sycamorenet.com>]

Ingo,

I tried just removing the CONFIG_PREEMPT_RT code, but that drops 
packets if another task has the lock.  Here's the debug printouts:

<4>xmit_lock_owner owned by sigd not softirq-net-rx/
<4>xmit_lock_owner owned by sigd not softirq-net-rx/
<4>xmit_lock_owner owned by sigd not softirq-net-rx/

Our quality department has been testing the patch below for a 
few days and has not seen any problems.  It pretty much 
preserves the original -rt patch pieces, but adds recursive checking.

I changed xmit_lock_owner to a void * as it is now a pointer
to the task which owns the lock.  What do you think?

Thanks,
	Mark

diff -ur linux-2.6.23-rc1-rt0/include/linux/netdevice.h linux-2.6.23-rc1-rt0_new/include/linux/netdevice.h
--- linux-2.6.23-rc1-rt0/include/linux/netdevice.h      2007-07-24 15:17:07.000000000 -0400
+++ linux-2.6.23-rc1-rt0_new/include/linux/netdevice.h  2007-08-01 09:01:32.000000000 -0400
@@ -468,7 +468,11 @@
        /* cpu id of processor entered to hard_start_xmit or -1,
           if nobody entered there.
         */
-       int                     xmit_lock_owner;
+#ifdef CONFIG_PREEMPT_RT
+       void *                  xmit_lock_owner;
+#else
+       int                     xmit_lock_owner;
+#endif
        void                    *priv;  /* pointer to private data      */
        int                     (*hard_start_xmit) (struct sk_buff *skb,
                                                    struct net_device *dev);
@@ -1041,32 +1045,54 @@
 static inline void netif_tx_lock(struct net_device *dev)
 {
        spin_lock(&dev->_xmit_lock);
-       dev->xmit_lock_owner = raw_smp_processor_id();
+#ifdef CONFIG_PREEMPT_RT
+       dev->xmit_lock_owner = (void *)current;
+#else
+       dev->xmit_lock_owner = raw_smp_processor_id();
+#endif
 }
 
 static inline void netif_tx_lock_bh(struct net_device *dev)
 {
        spin_lock_bh(&dev->_xmit_lock);
-       dev->xmit_lock_owner = raw_smp_processor_id();
+#ifdef CONFIG_PREEMPT_RT
+       dev->xmit_lock_owner = (void *)current;
+#else
+       dev->xmit_lock_owner = raw_smp_processor_id();
+#endif
 }
 
 static inline int netif_tx_trylock(struct net_device *dev)
 {
        int ok = spin_trylock(&dev->_xmit_lock);
        if (likely(ok))
-               dev->xmit_lock_owner = raw_smp_processor_id();
+       {
+#ifdef CONFIG_PREEMPT_RT
+       dev->xmit_lock_owner = (void *)current;
+#else
+       dev->xmit_lock_owner = raw_smp_processor_id();
+#endif
+    }
        return ok;
 }
 
 static inline void netif_tx_unlock(struct net_device *dev)
 {
+#ifdef CONFIG_PREEMPT_RT
        dev->xmit_lock_owner = -1;
+#else
+       dev->xmit_lock_owner = (void *)-1;
+#endif
        spin_unlock(&dev->_xmit_lock);
 }
 
 static inline void netif_tx_unlock_bh(struct net_device *dev)
 {
+#ifdef CONFIG_PREEMPT_RT
        dev->xmit_lock_owner = -1;
+#else
+       dev->xmit_lock_owner = (void *)-1;
+#endif
        spin_unlock_bh(&dev->_xmit_lock);
 }
 
diff -ur linux-2.6.23-rc1-rt0/net/core/dev.c linux-2.6.23-rc1-rt0_new/net/core/dev.c
--- linux-2.6.23-rc1-rt0/net/core/dev.c 2007-07-24 15:17:07.000000000 -0400
+++ linux-2.6.23-rc1-rt0_new/net/core/dev.c     2007-08-01 08:56:02.000000000 -0400
@@ -1592,7 +1592,7 @@
                 * No need to check for recursion with threaded interrupts:
                 */
 #ifdef CONFIG_PREEMPT_RT
-               if (1) {
+               if (dev->xmit_lock_owner != (void *)current) {
 #else
                int cpu = raw_smp_processor_id(); /* ok because BHs are off */
 
diff -ur linux-2.6.23-rc1-rt0/net/sched/sch_generic.c linux-2.6.23-rc1-rt0_new/net/sched/sch_generic.c
--- linux-2.6.23-rc1-rt0/net/sched/sch_generic.c        2007-07-24 15:17:07.000000000 -0400
+++ linux-2.6.23-rc1-rt0_new/net/sched/sch_generic.c    2007-08-01 08:57:14.000000000 -0400
@@ -153,7 +153,13 @@
 
        if (!lockless) {
 #ifdef CONFIG_PREEMPT_RT
-               netif_tx_lock(dev);
+        if (dev->xmit_lock_owner == (void *)current) {
+            kfree_skb(skb);
+            if (net_ratelimit())
+                printk(KERN_DEBUG "Dead loop on netdevice %s, fix it urgently!\n", dev->name);
+            return -1;
+        }
+        netif_tx_lock(dev);
 #else
                if (netif_tx_trylock(dev))
                        /* Another CPU grabbed the driver tx lock */



-----Original Message-----
From: Ingo Molnar [mailto:mingo@elte.hu]
Sent: Tuesday, July 24, 2007 3:15 PM
To: Beauchemin, Mark
Cc: Thomas Gleixner; linux-kernel@vger.kernel.org; David Miller
Subject: Re: [PATCH -rt] Preemption problem in kernel RT Patch



* Beauchemin, Mark <Mark.Beauchemin@sycamorenet.com> wrote:

> I'm not sure why the check for recursion has been removed.  In the 
> backtrace below, I think it would be caught by this check and not 
> recursively call the spinlock code.

ah ... i think i did it like that because i didnt realize that there 
would be a recursive call sequence, i was concentrating on recursive 
locking.

incidentally, this code got cleaned up in .23-rc1-rt0, and now it looks 
quite similar to your suggested fix. Could you double-check that it 
solves your problem?

	Ingo


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