netdev - Re: 3.11-rc6 genetlink locking fix offends lockdep

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Date:	Mon, 19 Aug 2013 13:22:42 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Ding Tianhong <dingtianhong@...wei.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	"Otcheretianski, Andrei" <andrei.otcheretianski@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Pravin B Shelar <pshelar@...ira.com>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: 3.11-rc6 genetlink locking fix offends lockdep

On Mon, 2013-08-19 at 19:00 +0800, Ding Tianhong wrote:
> On 2013/8/19 16:00, Johannes Berg wrote:
> > 
> >> 3.11-rc6's commit 58ad436fcf49 ("genetlink: fix family dump race")
> >> gives me the lockdep trace below at startup.
> > 
> > Hmm. Yes, I see now how this happens, not sure why I didn't run into it.
> > 
> > The problem is that genl_family_rcv_msg() is called with the genl_lock
> > held, and then calls netlink_dump_start() with it held, creating a
> > genl_lock->cb_mutex dependency, but obviously the dump continuation is
> > the other way around.
> > 
> > We could use the semaphore instead, I believe, but I don't really
> > understand the mutex vs. semaphore well enough to be sure that's
> > correct.
> > 
> > johannes
> > 
> it is useless, the logic need to modify or otherwise it will still call lockdep trace.

I don't believe so, the semaphore and cb_mutex don't have a dependency
yet, afaict.

> maybe i could send a patch for it, if you wish.

What do you mean?

johannes

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