Re: 2.6.13-rc6-rt6

Paul E. McKenney
2005-08-19T23:12:39Z

On Fri, Aug 19, 2005 at 07:02:42PM -0400, Steven Rostedt wrote:
> 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) {" :-)

Right you are! ;-)

> > 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.

Understood!

> > 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.

Hmmm.... Guess it is time for me to stop procrastinating on better
understanding git...

Thanx, Paul