Re: [PATCH v3 9/9] drm/tegra: Support context isolation

[Mikko Perttunen]

On 2/21/22 22:02, Dmitry Osipenko wrote:
> 21.02.2022 15:06, Mikko Perttunen пишет:
> > On 2/19/22 20:35, Dmitry Osipenko wrote:
> > > 18.02.2022 14:39, Mikko Perttunen пишет:
> > > > +    if (context->memory_context &&
> > > > context->client->ops->get_streamid_offset) {
> > >               ^^^
> > > > +        int offset =
> > > > context->client->ops->get_streamid_offset(context->client);
> > > > +
> > > > +        if (offset >= 0) {
> > > > +            job->context = context->memory_context;
> > > > +            job->engine_streamid_offset = offset;
> > > > +            host1x_context_get(job->context);
> > > > +        }
> > >
> > > You should bump refcount unconditionally or you'll get refcnt underflow
> > > on put, when offset < 0.
> >
> > This refcount is intended to be dropped from 'release_job', where it's
> > dropped if job->context is set, which it is from this path.
> >
> > > > +    }
> > > > +
> > > >        /*
> > > >         * job_data is now part of job reference counting, so don't
> > > > release
> > > >         * it from here.
> > > > diff --git a/drivers/gpu/drm/tegra/uapi.c b/drivers/gpu/drm/tegra/uapi.c
> > > > index 9ab9179d2026..be33da54d12c 100644
> > > > --- a/drivers/gpu/drm/tegra/uapi.c
> > > > +++ b/drivers/gpu/drm/tegra/uapi.c
> > > > @@ -33,6 +33,9 @@ static void tegra_drm_channel_context_close(struct
> > > > tegra_drm_context *context)
> > > >        struct tegra_drm_mapping *mapping;
> > > >        unsigned long id;
> > > >    +    if (context->memory_context)
> > > > +        host1x_context_put(context->memory_context);
> > >
> > > The "if (context->memory_context &&
> > > context->client->ops->get_streamid_offset)" above doesn't match the "if
> > > (context->memory_context)". You'll get refcount underflow.
> >
> > And this drop is for the refcount implicitly added when allocating the
> > memory context through host1x_context_alloc; so these two places should
> > be independent.
> >
> > Please elaborate if I missed something.
>
> You named context as memory_context and then have
> context=context->memory_context. Please try to improve the variable
> names, like drm_ctx->host1x_ctx for example.
>
> I'm also not a big fan of the "if (ref) put(ref)" pattern. Won't be
> better to move all the "if (!NULL)" checks inside of get()/put() and
> make the invocations unconditional?

I agree that the naming here is confusing with different kinds of 
contexts flying around, though I would prefer not to change the original 
'struct tegra_drm_context *context' since it's used all around the code. 
But I'll try to make it clearer.

Regarding moving NULL checks inside get/put, I personally dislike that 
pattern (also with e.g. kfree) since when reading the code, it makes it 
more difficult to see that the pointer can be NULL.

Mikko