On Sat, 27 Oct 2007 17:28:41 +0200 Peter Zijlstra <[email protected]> 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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