Re: [Bug #13819] system freeze when switching to console

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 8 Sep 2009, Linus Torvalds wrote:
> 
> The code here is
> 
> 	  16:	48 8b 80 00 01 00 00 	mov    0x100(%rax),%rax
> 	  1d:	48 8b 50 08          	mov    0x8(%rax),%rdx
> 	  21:	48 85 d2             	test   %rdx,%rdx
> 	  24:	74 11                	je     0x37
> 	  26:	49 8b 44 24 78       	mov    0x78(%r12),%rax
> 	  2b:*	8b 80 84 00 00 00    	mov    0x84(%rax),%eax     <-- trapping instruction
> 	  31:	89 82 08 08 00 00    	mov    %eax,0x808(%rdx)
> 	  37:	f6 45 a0 02          	testb  $0x2,-0x60(%rbp)
> 
> and that "testb $0x2, -0x60(%rbp)" seems to be the
> 
> 	if (iir & I915_USER_INTERRUPT) {

Yeah, that seems to be the right thing.

So the actual faulting instruction is from this:

                if (dev->primary->master) {
                        master_priv = dev->primary->master->driver_priv;
                        if (master_priv->sarea_priv)
                                master_priv->sarea_priv->last_dispatch =
					READ_BREADCRUMB(dev_priv);

and it looks like %rax starts out being 'dev', then the

	mov    0x100(%rax),%rax

means that %rax is now 'dev->primary', and then

	mov    0x8(%rax),%rdx

moves 'dev->primary->master' into %rdx. It's not zero, so we then do that 
READ_BREADCRUMB(dev_priv), which expands to

	READ_HWSP(dev_priv, I915_BREADCRUMB_INDEX)

which in turn is

	(((volatile u32*)(dev_priv->hw_status_page))[reg])

and it looks like dev_priv->hw_status_page is NULL.

You can verify this by looking at teh exception address:

	BUG: unable to handle kernel NULL pointer dereference at 0000000000000084

and that '84' is I915_BREADCRUMB_INDEX*4 (0x21*4).

And the problem seems to be that we've cleared the hw_status_page pointer 
in i915_gem_cleanup_hws():

	dev_priv->hw_status_page = NULL;

and we did that in 

  i915_gem_idle() ->
    i915_gem_cleanup_ringbuffer() ->
      i915_gem_cleanup_hws()

so now since interrupts are still enabled, you'll get a NULL pointer 
dereference.

I think my patch is correct.

		Linus