Subject: Re: mmap()ed AF_NETLINK: lockdep and sleep-in-atomic warnings

From: Florian Westphal <[email-masked]>
Date: Tue, 14 Jul 2015 11:50:37 +0200

Thomas Graf <[email-masked]> wrote:
> On 07/13/15 at 10:11pm, Cong Wang wrote:
> > Caused by:
> > 
> > commit 21e4902aea80ef35afc00ee8d2abdea4f519b7f7
> > Author: Thomas Graf <[email-masked]>
> > Date:   Fri Jan 2 23:00:22 2015 +0100
> > 
> >     netlink: Lockless lookup with RCU grace period in socket release
> > 
> >     Defers the release of the socket reference using call_rcu() to
> >     allow using an RCU read-side protected call to rhashtable_lookup()
> > 
> >     This restores behaviour and performance gains as previously
> >     introduced by e341694 ("netlink: Convert netlink_lookup() to use
> >     RCU protected hash table") without the side effect of severely
> >     delayed socket destruction.
> > 
> >     Signed-off-by: Thomas Graf <[email-masked]>
> >     Signed-off-by: David S. Miller <[email-masked]>
> > 
> > 
> > We can't hold mutex lock in a rcu callback, perhaps we could
> > defer the mmap ring cleanup to a workqueue.
> 
> The socket should be dead at this point. It might be simpler to
> add a netlink_release_ring() function which doesn't require
> locking at all. It would also get rid of all the special handling
> for closing vs. non-closing.

Do you know of any users of mmap'd netlink?
Support for this was never added to libmnl; given the fact that netlink
mmap doesn't have any advantages vs. socket interface i'd just revert it
(or mark CONFIG_NETLINK_MMAP broken -- although I don't see
 how the unconditional copy on tx could ever be avoided).