Re: netpoll rtnl_dereference() usage

From: Neil Horman
Date: Fri Mar 25 2016 - 14:16:17 EST

On Fri, Mar 25, 2016 at 12:30:32PM -0500, Bjorn Helgaas wrote:
> 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().
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.

> 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.
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


> Bjorn