linux-kernel - Re: BUG: lock held when returning to user space

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Date:	Sat, 27 Oct 2007 08:46:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Jiri Kosina <jikos@...os.cz>,
	Gabriel C <nix.or.die@...glemail.com>, a.zummo@...ertech.it,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	rtc-linux@...glegroups.com, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: BUG: lock held when returning to user space

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