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

[Bjorn Andersson]

On Tue 01 Sep 03:42 UTC 2020, Rob Clark wrote:

> 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
> 

Looks reasonable, thanks for the explanation.

> 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
> > >