Re: 2.6.13-rc6-rt6

Steven Rostedt
2005-08-19T21:22:41Z

On Wed, 2005-08-17 at 18:23 +0200, Ingo Molnar wrote:

>
> > And it goes on and on. This happens everytime. Without netconsole, I
> > only get the nonzero lock count error. Also, one of my lockups on SMP
> > had to do with the kernel_thread_helper:
> >
> > Using IPI Shortcut mode
> > khelper/794[CPU#0]: BUG in set_new_owner at kernel/rt.c:916

This was with netconsole and showed up after a bunch of other bugs. So
this is a side effect of what happened earlier.
>
> this is a 'must not happen'. Somehow lock->held list got non-empty.
> Maybe some use-after-free thing? Havent seen it myself.
>

I started debugging netconsole with the RT patch and found this
happening. After seeing what's wrong, I looked at the latest git
branch, and it seems to already have a similar solution that I was going
to make. Here's a description of what's wrong.

In net/core/dev.c the following code is in net_rx_action:

netpoll_poll_lock(dev);

if (dev->quota <= 0 || dev->poll(dev, &budget)) {
netpoll_poll_unlock(dev);
raw_local_irq_disable();
list_del(&dev->poll_list);
list_add_tail(&dev->poll_list, &queue->poll_list);
if (dev->quota < 0)
dev->quota += dev->weight;
else
dev->quota = dev->weight;
} else {
netpoll_poll_unlock(dev);

The netpoll_poll_lock and netpoll_poll_unlock look like this (in current RT):

static inline netpoll_poll_lock(struct net_device *dev)
{
if (dev->npinfo) {
spin_lock(&dev->npinfo->poll_lock);
dev->npinfo->poll_owner = smp_processor_id();
}
}

static inline void netpoll_poll_unlock(struct net_device *dev)
{
if (dev->npinfo) {
dev->npinfo->poll_owner = -1;
spin_unlock(&dev->npinfo->poll_lock);
}
}

The problem here is that between netpoll_poll_lock and
netpoll_poll_unlock the dev->npinfo gets assigned. So we unlock the
dev->npinfo->poll_lock without ever locking it.

Here's the port from the latest git to solve this. I've CCed the netdev,
since I'm not sure I got all the places for rcu_lock for the netpoll. At
least to solve this problem. I did boot up the kernel and this patch
did fix my bugs that I was getting using netconsole. (I have one more
patch to send to fix the illegal API messages).

-- Steve

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Index: linux_realtime_ernie/include/linux/netpoll.h
===================================================================
--- linux_realtime_ernie/include/linux/netpoll.h (revision 296)
+++ linux_realtime_ernie/include/linux/netpoll.h (working copy)
@@ -60,25 +60,31 @@
return ret;
}

-static inline void netpoll_poll_lock(struct net_device *dev)
+static inline void *netpoll_poll_lock(struct net_device *dev)
{
+ rcu_read_lock();
if (dev->npinfo) {
spin_lock(&dev->npinfo->poll_lock);
dev->npinfo->poll_owner = smp_processor_id();
+ return dev->npinfo;
}
+ return NULL;
}

-static inline void netpoll_poll_unlock(struct net_device *dev)
+static inline void netpoll_poll_unlock(void *have)
{
- if (dev->npinfo) {
- dev->npinfo->poll_owner = -1;
- spin_unlock(&dev->npinfo->poll_lock);
+ struct netpoll_info *npi = have;
+
+ if (npi) {
+ npi->poll_owner = -1;
+ spin_unlock(&npi->poll_lock);
}
+ rcu_read_unlock();
}

#else
#define netpoll_rx(a) 0
-#define netpoll_poll_lock(a)
+#define netpoll_poll_lock(a) 0
#define netpoll_poll_unlock(a)
#endif

Index: linux_realtime_ernie/net/core/netpoll.c
===================================================================
--- linux_realtime_ernie/net/core/netpoll.c (revision 296)
+++ linux_realtime_ernie/net/core/netpoll.c (working copy)
@@ -726,6 +726,9 @@
/* last thing to do is link it to the net device structure */
ndev->npinfo = npinfo;

+ /* avoid racing with NAPI reading npinfo */
+ synchronize_rcu();
+
return 0;

release:
Index: linux_realtime_ernie/net/core/dev.c
===================================================================
--- linux_realtime_ernie/net/core/dev.c (revision 296)
+++ linux_realtime_ernie/net/core/dev.c (working copy)
@@ -1723,6 +1723,7 @@

while (!list_empty(&queue->poll_list)) {
struct net_device *dev;
+ void *have;

if (budget <= 0 || jiffies - start_time > 1)
goto softnet_break;
@@ -1735,10 +1736,10 @@

dev = list_entry(queue->poll_list.next,
struct net_device, poll_list);
- netpoll_poll_lock(dev);
+ have = netpoll_poll_lock(dev);

if (dev->quota <= 0 || dev->poll(dev, &budget)) {
- netpoll_poll_unlock(dev);
+ netpoll_poll_unlock(have);
raw_local_irq_disable();
list_del(&dev->poll_list);
list_add_tail(&dev->poll_list, &queue->poll_list);
@@ -1747,7 +1748,7 @@
else
dev->quota = dev->weight;
} else {
- netpoll_poll_unlock(dev);
+ netpoll_poll_unlock(have);
dev_put(dev);
raw_local_irq_disable();
}