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

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Date:	Tue, 14 Jul 2015 11:38:25 +0200
From:	Thomas Graf <tgraf@...g.ch>
To:	Cong Wang <cwang@...pensource.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: mmap()ed AF_NETLINK: lockdep and sleep-in-atomic warnings

On 07/13/15 at 10:11pm, Cong Wang wrote:
> Caused by:
> 
> commit 21e4902aea80ef35afc00ee8d2abdea4f519b7f7
> Author: Thomas Graf <tgraf@...g.ch>
> 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 <tgraf@...g.ch>
>     Signed-off-by: David S. Miller <davem@...emloft.net>
> 
> 
> 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.
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