Return-Path: <linux-kernel-owner+w=401wt.eu-S1755426AbXJ0PsL@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755426AbXJ0PsL (ORCPT <rfc822;w@1wt.eu>);
	Sat, 27 Oct 2007 11:48:11 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752719AbXJ0Pr6
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Sat, 27 Oct 2007 11:47:58 -0400
Received: from smtp2.linux-foundation.org ([207.189.120.14]:51177 "EHLO
	smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1752227AbXJ0Pr5 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sat, 27 Oct 2007 11:47:57 -0400
Date: Sat, 27 Oct 2007 08:46:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Kosina <jikos@jikos.cz>, Gabriel C <nix.or.die@googlemail.com>,
       a.zummo@towertech.it,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       rtc-linux@googlegroups.com, Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: BUG: lock held when returning to user space
Message-Id: <20071027084646.434ccb4f.akpm@linux-foundation.org>
In-Reply-To: <1193498921.5648.68.camel@lappy>
References: <472348F6.6050100@googlemail.com>
	<Pine.LNX.4.64.0710271707140.18815@twin.jikos.cz>
	<1193498921.5648.68.camel@lappy>
X-Mailer: Sylpheed 2.4.1 (GTK+ 2.8.17; x86_64-unknown-linux-gnu)
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2152
Lines: 46

On Sat, 27 Oct 2007 17:28:41 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> 
> On Sat, 2007-10-27 at 17:12 +0200, Jiri Kosina wrote:
> > On Sat, 27 Oct 2007, Gabriel C wrote:
> > 
> > > I found that today in dmesg after booting current git ( 
> > > ec3b67c11df42362ccda81261d62829042f223f0 ) :
> > > ...
> > > [  592.752777]
> > > [  592.752781] ================================================
> > > [  592.753478] [ BUG: lock held when returning to user space! ]
> > > [  592.753880] ------------------------------------------------
> > > [  592.754262] hwclock/1452 is leaving the kernel with locks still held!
> > > [  592.754655] 1 lock held by hwclock/1452:
> > > [  592.755007]  #0:  (&rtc->char_lock){--..}, at: [<c02a7ebb>] rtc_dev_open+0x2e/0x7e                                        
> > 
> > Yes, this is because rtc keeps a char_lock mutex locked as long as the 
> > device is open, to avoid concurrent accessess.
> > 
> > It could be easily substituted by some counting -- setting and clearing 
> > bit in struct rtc_device instead of using char_lock, but doing this just 
> > to shut the lockdep off is questionable imho.
> > 
> > Peter, what is the preferred way to annotate these kinds of locking for 
> > lockdep to express that it is intended?
> 
> Not sure, I'd not thought that anyone would actually want to do this.
> I'm also not sure how I stand on this, I'd prefer to say: don't do this!
> 
> I think, in this case, the lock is associated with a kernel object that
> is properly cleaned up if the holding tasks gets a SIGKILL. But in
> general I'd like to see this kind of thing go away.
> 
> Now I could probably come up with an annotation to hide it, but what do
> other people think, Ingo, Linus, Andrew, do we want to keep kernel locks
> held over userspace?
> 

It's a fairly daft thing to do.  I think it'd be saner to teach rtc about
test_and_set_bit() personally..
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/