RT patches expose netdev race [was Re: [RFC] [patch 2/2] powerpc 2.6.21-rt1: fix kernel hang and/or panic

[Linas Vepstas]

On Wed, May 16, 2007 at 05:41:01PM -0700, David Miller wrote:
> From: linas at austin.ibm.com (Linas Vepstas)
> Date: Wed, 16 May 2007 19:18:02 -0500
> 
> > Since this is a long email; let me put a summary up front:
> > I think the RT/premption patches are exposing some sort
> > of race in the ip header handling code. The rest of the 
> > note is forensics pointing to this.
> 
> skb->head should never ever be NULL.

The stack trace from Owa-san showed a null pointer deref at 
ip_hdr(skb)->protocol for an skb passed in via hard_start_xmit()

I dunno, memory corruption?

Tsutomu, can you reproduce this with something similr to the following
patch?

--linas

 drivers/net/spider_net.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

Index: linux-2.6.22-rc1/drivers/net/spider_net.c
===================================================================
--- linux-2.6.22-rc1.orig/drivers/net/spider_net.c	2007-05-17 18:31:40.000000000 -0500
+++ linux-2.6.22-rc1/drivers/net/spider_net.c	2007-05-17 18:51:49.000000000 -0500
@@ -720,7 +720,19 @@ spider_net_prepare_tx_descr(struct spide
 			SPIDER_NET_DESCR_CARDOWNED | SPIDER_NET_DMAC_NOCS;
 	spin_unlock_irqrestore(&chain->lock, flags);
 
-	if (skb->protocol == htons(ETH_P_IP) && skb->ip_summed == CHECKSUM_PARTIAL)
+	if (skb->protocol == htons(ETH_P_IP) && skb->ip_summed == CHECKSUM_PARTIAL) {
+		struct iphdr *hp=ip_hdr(skb);
+		if (((unsigned long) hp < 0x100000) || 
+		    ((unsigned long)hp > 0xffff000000000000UL)) {
+			printk(KERN_ERROR "spidernet: bad ip header! "
+				"skb=%p ip_hdr=%p head=%p data=%p net=%x\n", skb, hp,
+				skb->head, skb->data, skb->network_header);
+			int i;
+			unsinged long *s=(unsigned long*) skb;
+			for (i=0; i<20; i++) {
+				printk("%d %lx %lx\n", i, s[2*i],s[2*i+1]);
+			}
+		} else {
 		switch (ip_hdr(skb)->protocol) {
 		case IPPROTO_TCP:
 			hwdescr->dmac_cmd_status |= SPIDER_NET_DMAC_TCP;
@@ -728,6 +740,8 @@ spider_net_prepare_tx_descr(struct spide
 		case IPPROTO_UDP:
 			hwdescr->dmac_cmd_status |= SPIDER_NET_DMAC_UDP;
 			break;
+		}
+		}
 	}
 
 	/* Chain the bus address, so that the DMA engine finds this descr. */