Re: [uml-devel] 2.6.25 uml kernel crashes when it calls down() on a semaphore with zero counter

[Mattia Dongili <malattia@linux.it>]

On Fri, Jul 18, 2008 at 04:53:42PM -0400, Jeff Dike wrote:
> On Thu, Jul 17, 2008 at 12:55:09PM +0800, Jiaying Zhang wrote:
> > The patch below solves the 2.6.25 uml crash problem for me. Looks like the
> > problem should be away in 2.6.26 kernel because down_interruptible has
> > changed to the C code since 2.6.26. But I got kernel panic while booting
> > the 2.6.26 kernel :(.
> > 
> > --- linux-2.6.25.4/lib/semaphore-sleepers.c     2008-05-15
> > 23:00:12.000000000 +0800
> > +++ linux-2.6.25.4-new/lib/semaphore-sleepers.c 2008-07-17
> > 12:20:47.000000000 +0800
> > @@ -48,12 +48,12 @@
> >   *    we cannot lose wakeup events.
> >   */
> > 
> > -void __up(struct semaphore *sem)
> > +asmregparm void __up(struct semaphore *sem)
> >  {
> >         wake_up(&sem->wait);
> >  }
> 
> You continue to ignore a few important facts:
> 
>     1 - There are a ton of semaphores in UML
>     2 - They all work, except for yours
>     Therefore, a patch which changes all semphores across all
> architectures for which asmregparam has meaning can't possibly be the
> correct fix.
> 
> However, you might have treated this as an important clue, and looked
> at whether your broken semaphore has a different set of declarations
> in force than those in the rest of the kernel.

Jeff,
it's not entirely clear to me why, but that patch fixes a segfault that
I experience when booting uml 2.6.25 built with gcc-4.3 on a 2.6.25
host (I also applied your ICE workaround patch).
I'm booting a debian sid image that I usually run before uploading the
new uml package in debian.
I've got no fancy modules written by me and the segfault is 100%
reproducible with that debian image (a different image -a gentoo-
doesn't crash).

I'll provide more info tomorrow, I'll try to further trace the crash
with gdb.

cheers
-- 
mattia
:wq!

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