[Intel-gfx] [PATCH 02/13] drm/i915: fix invalid reference handling of the default ctx obj

[Chris Wilson]

On Fri, 13 Jul 2012 17:37:14 +0200, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Fri, Jul 13, 2012 at 02:14:05PM +0100, Chris Wilson wrote:
> > Otherwise we end up trying to unpin a freed object and BUG.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Ben Widawsky <ben at bwidawsk.net>
> 
> Afact this patch contains quite some code refactoring that does not
> relate directly to the fix (or if it does, I fail to see the direct
> relevance). So I think this either needs an explanation in the commit
> message or be put into a separate patch (I agree though for actual code
> cleanups).
> 
> For the fix itself I seem to be a bit dense again - the only thing I see
> is that you move the refcount handling into do_switch. Afacs we do the
> ref-handling in both cases only when do_switch is successful, and also
> right at the end of do_switch (or right afterwards). So can you please
> enlighten your clueless maintainer a bit an explain how things blow up?

The fix is that the reference handling was only done on one path, not
both. Hence the default_ctx ends up being used-after-free.

The rest of it was just unwinding the code to get to finding the bug...
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre