Re: [PATCH 01/19] drm/msm: remove dangling submitqueue references

[Rob Clark]

On Mon, Aug 31, 2020 at 7:35 PM Bjorn Andersson
<bjorn.andersson@xxxxxxxxxx> wrote:
>
> On Fri 14 Aug 02:40 UTC 2020, Rob Clark wrote:
>
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > Currently it doesn't matter, since we free the ctx immediately.  But
> > when we start refcnt'ing the ctx, we don't want old dangling list
> > entries to hang around.
> >
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/msm/msm_submitqueue.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/msm/msm_submitqueue.c b/drivers/gpu/drm/msm/msm_submitqueue.c
> > index a1d94be7883a..90c9d84e6155 100644
> > --- a/drivers/gpu/drm/msm/msm_submitqueue.c
> > +++ b/drivers/gpu/drm/msm/msm_submitqueue.c
> > @@ -49,8 +49,10 @@ void msm_submitqueue_close(struct msm_file_private *ctx)
> >        * No lock needed in close and there won't
> >        * be any more user ioctls coming our way
> >        */
> > -     list_for_each_entry_safe(entry, tmp, &ctx->submitqueues, node)
> > +     list_for_each_entry_safe(entry, tmp, &ctx->submitqueues, node) {
> > +             list_del(&entry->node);
>
> If you refcount ctx, what does that do for the entries in the submit
> queue?
>
> "entry" here is kref'ed, but you're popping it off the list regardless
> of the put ends up freeing the object or not - which afaict would mean
> leaking the object.
>

What ends up happening is the submit has reference to submit-queue,
which has reference to the ctx.. the submitqueue could be alive still
pending in-flight submits (in a later patch), but dead from the PoV of
userspace interface.

We aren't relying (or at least aren't in the end, and I *think* I
didn't miss anything in the middle) relying on ctx->submitqueues list
to clean anything up in the end, just track what is still a valid
submitqueue from userspace PoV

BR,
-R

>
> On the other hand, with the current implementation an object with higher
> refcount with adjacent objects of single refcount would end up with
> dangling pointers after the put. So in itself this change seems like a
> net gain, but I'm wondering about the plan described in the commit
> message.
>
> Regards,
> Bjorn
>
> >               msm_submitqueue_put(entry);
> > +     }
> >  }
> >
> >  int msm_submitqueue_create(struct drm_device *drm, struct msm_file_private *ctx,
> > --
> > 2.26.2
> >