Re: do_IRQ: stack overflow: 872..

David Woodhouse
2005-01-07T17:12:37Z

On Sat, 2004-12-18 at 08:50 +0100, Andi Kleen wrote:
> It's not really an oops, just a warning that stack space got quiet
> tight.
>
> The problem seems to be that the br netfilter code is nesting far too
> deeply and recursing several times. Looks like a design bug to me,
> it shouldn't do that.

I don't think it's recursing -- I think the stack trace is just a bit
noisy. The problem is that the bridge code, especially with br_netfilter
in the equation, is implicated in code paths which are just _too_ deep.
This happens when you're bridging packets received in an interrupt while
you were deep in journalling code, and it's also been seen with a call
trace something like nfs->sunrpc->ip->bridge->br_netfilter.

One option might be to make br_dev_xmit() just queue the packet rather
than trying to deliver it to all the slave devices immediately. Then the
actual retransmission can be handled from a context where we're _not_
short of stack; perhaps from a dedicated kernel thread.

Unfortunately that approach would introduce a lot of latency on all
packets we pass. Another option would be to have all architectures
provide a stack_available() function and for br_dev_xmit() to queue the
packet only if we're short of stack, while still sending most packets
immediately.

Proof of concept below; obviously the stack_available() is an evil hack
and would need to be done more sanely. Comments?

===== net/bridge/br_device.c 1.17 vs edited =====
--- 1.17/net/bridge/br_device.c 2004-07-29 22:40:51 +01:00
+++ edited/net/bridge/br_device.c 2005-01-07 16:54:26 +00:00
@@ -19,6 +19,13 @@
#include <asm/uaccess.h>
#include "br_private.h"

+static inline unsigned long stack_available(void)
+{
+ unsigned long esp;
+ asm volatile("movl %%esp,%0" : "=r"(esp));
+ return esp - (unsigned long)current - sizeof(struct thread_info);
+}
+
static struct net_device_stats *br_dev_get_stats(struct net_device *dev)
{
struct net_bridge *br;
@@ -34,6 +41,14 @@
const unsigned char *dest = skb->data;
struct net_bridge_fdb_entry *dst;

+ if (stack_available() < THREAD_SIZE/2) {
+ if (net_ratelimit()) {
+ printk(KERN_DEBUG "Bridge device %s queues packet due to stack shortage\n",
+ dev->name);
+ }
+ return NETDEV_TX_BUSY;
+ }
+
br->statistics.tx_packets++;
br->statistics.tx_bytes += skb->len;

@@ -104,7 +119,7 @@
SET_MODULE_OWNER(dev);
dev->stop = br_dev_stop;
dev->accept_fastpath = br_dev_accept_fastpath;
- dev->tx_queue_len = 0;
+ dev->tx_queue_len = 5;
dev->set_mac_address = NULL;
dev->priv_flags = IFF_EBRIDGE;
}

--
dwmw2