Re: [PATCH v3 2/4] drm/qxl: unpin release objects

[Daniel Vetter]

On Fri, Jan 22, 2021 at 2:35 PM Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
>
> On Fri, Jan 22, 2021 at 09:13:42AM +0100, Thomas Zimmermann wrote:
> > Hi
> >
> > Am 20.01.21 um 12:12 schrieb Gerd Hoffmann:
> > > Balances the qxl_create_bo(..., pinned=true, ...);
> > > call in qxl_release_bo_alloc().
> > >
> > > Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/qxl/qxl_release.c | 1 +
> > >   1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/gpu/drm/qxl/qxl_release.c b/drivers/gpu/drm/qxl/qxl_release.c
> > > index 0fcfc952d5e9..add979cba11b 100644
> > > --- a/drivers/gpu/drm/qxl/qxl_release.c
> > > +++ b/drivers/gpu/drm/qxl/qxl_release.c
> > > @@ -166,6 +166,7 @@ qxl_release_free_list(struct qxl_release *release)
> > >             entry = container_of(release->bos.next,
> > >                                  struct qxl_bo_list, tv.head);
> > >             bo = to_qxl_bo(entry->tv.bo);
> > > +           bo->tbo.pin_count = 0; /* ttm_bo_unpin(&bo->tbo); */
> >
> > This code looks like a workaround or a bug.
> >
> > AFAICT the only place with pre-pinned BO is qdev->dumb_shadow_bo. Can you
> > remove the pinned flag entirely and handle pinning as part of
> > dumb_shadow_bo's code.
>
> No, the release objects are pinned too, and they must be
> pinned (qxl commands are in there, and references are
> placed in the qxl rings, so allowing them to roam is
> a non-starter).
>
> > if (pin_count)
> >     ttm_bo_unpin();
> > WARN_ON(pin_count); /* should always be 0 now */
>
> Well, the pin_count is 1 at this point.
> No need for the if().
>
> Just calling ttm_bo_unpin() here makes lockdep unhappy.

How does that one splat? But yeah if that's a problem should be
explained in the comment. I'd then also only do a pin_count--; to make
sure you can still catch other pin leaks if you have them. Setting it
to 0 kinda defeats the warning.
-Daniel

>
> Not calling ttm_bo_unpin() makes ttm_bo_release() throw
> a WARN() because of the pin.
>
> Clearing pin_count (which is how ttm fixes things up
> in the error path) works.
>
> I'm open to better ideas.
>
> take care,
>   Gerd
>
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch