netpoll rtnl_dereference() usage

Hi Neil,

Since we're looking at netpoll, here's another question (or two). 0790bbb68f9d ("netpoll: cleanup sparse warnings") adds this:

@@ -1236,7 +1236,11 @@ void __netpoll_cleanup(struct netpoll *np)

        struct netpoll_info *npinfo;
        unsigned long flags;

- npinfo = np->dev->npinfo;
+ /* rtnl_dereference would be preferable here but

+        * rcu_cleanup_netpoll path can put us in here safely without
+        * holding the rtnl, so plain rcu_dereference it is
+        */
+       npinfo = rtnl_dereference(np->dev->npinfo);
        if (!npinfo)
                return;

The comment seems to contradict the code: the comment says "we would like to use rtnl_dereference(), but we have to use rcu_dereference()." But the code in fact *does* use rtnl_dereference().

Also, "rcu_cleanup_netpoll" doesn't exist; maybe it's a typo for rcu_cleanup_netpoll_info()? I don't see the path that leads from rcu_cleanup_netpoll_info() to __netpoll_cleanup(), but I don't claim to understand the netpoll async subtleties.

Bjorn

its the comment that went awry. I remember writing that patch, and I initially thought we had to use rcu_derefence there, but I would up finding a way to keep the rntl lock held, so rtnl_deref should be ok. I must have just forgotten to fixup the comment.

Correct again, its the rcu callback rcu_cleanup_netpoll_info that I'm referring to there, and the comment was written initially when rcu_cleanup_netpoll info was called cleanup_netpoll_info and called forward into __netpoll_cleanup (in my development patch versions). That comment should really just be re-written. I'm happy to do so if you like

Best
Neil