linux-kernel - Re: 2.6.30-rc6: Reported regressions from 2.6.29

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Date:	Sun, 17 May 2009 08:32:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: 2.6.30-rc6: Reported regressions from 2.6.29


* Kay Sievers <kay.sievers@...y.org> wrote:

> On Sun, 2009-05-17 at 02:15 +0200, Kay Sievers wrote:
> > On Sun, May 17, 2009 at 02:01, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> > 
> > >        [   10.428691] Error: Driver 'rtc_cmos' is already registered, aborting...
> > >
> > > but the driver doesn't really care whether that succeeded or not, but then
> > > the platform_driver_probe fails (because the thing is already in use), so
> > > then it ends up unregistering something that never got registered in the
> > > first place.
> > >
> > > I think this is strictly speaking a bug in driver_unregister(), which is
> > > too fragile. If you unregister a drivert that wasn't registered, we
> > > shouldn't oops.
> > >
> > > But we could certainly do it at the rtc-cmos.c level too, and just not
> > > unregister it if the registration failed. My gut feel is that we should
> > > aim for the core driver helpers to be less fragile, though - we'll always
> > > have driver bugs.
> > 
> > In:
> >  driver_remove_file()
> > we try to access the private part:
> >  sysfs_remove_file(&drv->p->kobj, ...
> > and that is NULL, for an unregistered driver, I would expect.
> > 
> > I'm looking into it.
> 
> This makes the oops in the driver-core, caused by the rtc driver
> unregister, go away. The original issue is also fixed in the rtc driver
> itself.
> 
> Thanks,
> Kay
> 
> 
> From: Kay Sievers <kay.sievers@...y.org>
> Subject: Driver Core: do not oops when driver_unregister() is called for unregistered drivers
> 
> Handle the case someone tries to unregister a non-registered driver
> more gracefully.
> 
>   Error: Driver 'rtc_cmos' is already registered, aborting...
>   BUG: unable to handle kernel NULL pointer dereference at 00000018
>   [<c01e16ba>] sysfs_remove_file+0x1/0xf
> 
> Cc: Greg Kroah-Hartman <gregkh@...e.de>
> Signed-off-by: Kay Sievers <kay.sievers@...y.org>

please also put in a proper Reported-by.

> ---
> 
>  driver.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> --- a/drivers/base/driver.c
> +++ b/drivers/base/driver.c
> @@ -257,6 +257,8 @@ EXPORT_SYMBOL_GPL(driver_register);
>   */
>  void driver_unregister(struct device_driver *drv)
>  {
> +	if (!drv || !drv->p)
> +		return;

I think it would be reasonable to also emit a:

	 WARN_ONCE(1, "unexpected driver unregister!\n");

here - as long as all such cases are a bug. As the imbalance (and a 
bug) is really at the rtc-cmos driver level too and we should not 
condone such pattern, silently.

( Not crashing in the driver core when we can avoid it is nice to 
  have too, of course. )

	Ingo
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