Re: [Intel-gfx] [PATCH] drm/i915/gt: Fix -EDEADLK handling regression

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jul 1, 2021 at 9:07 AM Maarten Lankhorst
<maarten.lankhorst@linux.intel.com> wrote:
> Op 30-06-2021 om 18:44 schreef Ville Syrjala:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > The conversion to ww mutexes failed to address the fence code which
> > already returns -EDEADLK when we run out of fences. Ww mutexes on
> > the other hand treat -EDEADLK as an internal errno value indicating
> > a need to restart the operation due to a deadlock. So now when the
> > fence code returns -EDEADLK the higher level code erroneously
> > restarts everything instead of returning the error to userspace
> > as is expected.
> >
> > To remedy this let's switch the fence code to use a different errno
> > value for this. -ENOBUFS seems like a semi-reasonable unique choice.
> > Apart from igt the only user of this I could find is sna, and even
> > there all we do is dump the current fence registers from debugfs
> > into the X server log. So no user visible functionality is affected.
> > If we really cared about preserving this we could of course convert
> > back to -EDEADLK higher up, but doesn't seem like that's worth
> > the hassle here.
> >
> > Not quite sure which commit specifically broke this, but I'll
> > just attribute it to the general gem ww mutex work.
> >
> > Cc: stable@vger.kernel.org
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Thomas Hellström <thomas.hellstrom@intel.com>
> > Testcase: igt/gem_pread/exhaustion
> > Testcase: igt/gem_pwrite/basic-exhaustion
> > Testcase: igt/gem_fenced_exec_thrash/too-many-fences
> > Fixes: 80f0b679d6f0 ("drm/i915: Add an implementation for i915_gem_ww_ctx locking, v2.")
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c b/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
> > index cac7f3f44642..f8948de72036 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
> > @@ -348,7 +348,7 @@ static struct i915_fence_reg *fence_find(struct i915_ggtt *ggtt)
> >       if (intel_has_pending_fb_unpin(ggtt->vm.i915))
> >               return ERR_PTR(-EAGAIN);
> >
> > -     return ERR_PTR(-EDEADLK);
> > +     return ERR_PTR(-ENOBUFS);
> >  }
> >
> >  int __i915_vma_pin_fence(struct i915_vma *vma)
>
> Makes sense..
>
> Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>
> Is it a slightly more reent commit? Might probably be the part that converts execbuffer to use ww locks.

- please cc: dri-devel on anything gem/gt related.
- this should probably be ENOSPC or something like that for at least a
seeming retention of errno consistentcy:

https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#recommended-ioctl-return-values

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch