Quoting Tvrtko Ursulin (2018-02-19 18:31:31)
>
> On 19/02/2018 14:01, Chris Wilson wrote:
> > If we fail to unbind the vma (due to a signal on an active buffer that
> > needs to be moved for the next execbuf), then we need to clear the
> > persistent tracking state we setup for this execbuf.
> >
> > Fixes: c7c6e46f913b ("drm/i915: Convert execbuf to use struct-of-array
> > packing for critical fields")
> > Testcase: igt/gem_fenced_exec_thrash/no-spare-fences-busy*
> > Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> > Cc: Joonas Lahtinen <joonas.lahti...@linux.intel.com>
> > Cc: <sta...@vger.kernel.org> # v4.14+
> > ---
> > drivers/gpu/drm/i915/i915_gem_execbuffer.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > index 51f3c32c64bf..4eb28e84fda4 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > @@ -505,6 +505,8 @@ eb_add_vma(struct i915_execbuffer *eb, unsigned int i,
> > struct i915_vma *vma)
> > list_add_tail(&vma->exec_link, &eb->unbound);
> > if (drm_mm_node_allocated(&vma->node))
> > err = i915_vma_unbind(vma);
> > + if (unlikely(err))
> > + vma->exec_flags = NULL;
> > }
> > return err;
> > }
> >
>
> I was trying to track down what actually explodes for like 15 minutes.
>
> My track was:
>
> eb_relocate -> eb_lookup_vmas fails -> eb_relocate -> eb_relocate_slow
> -> eb_reset_vmas -> second pass to eb_lookup_vmas -> resets
> vma->exec_flags. So no explosion.
>
> So in other words I've failed to find what goes wrong and under which
> circumstances.
The first eb_relocate calls eb_lookup_vma triggers the failure and exit
from execbuf. In that path, we mark the current index as the sentinel
(err_vma: eb->vma[i] = NULL) which means we do not clear the last vma
when unwinding in eb_release_vmas. So the vma->exec_flags was carried
over into the next execbuf call from userspace.
-Chris
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