Re: 2.6.13-rc6-rt6

Steven Rostedt
2005-08-19T23:02:54Z

On Fri, 2005-08-19 at 15:47 -0700, Paul E. McKenney wrote:

> Good catch -- but a few changes needed to be perfectly safe:
>
> static inline void *netpoll_poll_lock(struct net_device *dev)
> {
>
> struct netpoll_info *npi;
>
> rcu_read_lock();
> npi = rcu_dereference(dev)->npinfo;
> if (have) {

Here I'm sure you mean "if (npi) {" :-)

> spin_lock(&npi->poll_lock);
> npi->poll_owner = smp_processor_id();
> return npi;
> }
> return NULL;
> }
>
> The earlier version could get in trouble if dev->npinfo was set
> to NULL while this was executing.

Truth be told, I was just fixing the race with getting the npinfo
pointer set between netpoll_poll_lock and netpoll_poll_unlock. I wrote
a patch that fixed that but nothing with the rcu_locks. Then I looked
at the current git tree and saw that they already had my changes, but
also included the rcu locks. So I just (blindly) added them.

>
> Again, I do not fully understand this code, so a grain of salt might
> come in handy. But there definitely need to be some rcu_dereference()
> and rcu_assign_pointer() primitives in there somewhere. ;-)
>
> The following changes look good to me, but, as I said earlier, I do
> not claim to fully understand this code.

netpoll has changed quite a bit in the last few releases. I've seen lots
of fixup code sent in (which usually means there's lots of new broken
code ;-)

Anyway, I don't quite fully understand RCU. I read a few of the
documents on your web site, but I haven't had time to really digest it.
Have you taken a look at the latest git tree? The rcu_locks are used
for net poll quite a bit more there.

-- Steve