[PATCH] drm/msm: Don't init ww_mutec acquire ctx before needed

[Rob Clark]

On Wed, Nov 20, 2019 at 2:56 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> For locking semantics it really doesn't matter when we grab the
> ticket. But for lockdep validation it does: the acquire ctx is a fake
> lockdep. Since other drivers might want to do a full multi-lock dance
> in their fault-handler, not just lock a single dma_resv. Therefore we
> must init the acquire_ctx only after we've done all the copy_*_user or
> anything else that might trigger a pagefault. For msm this means we
> need to move it past submit_lookup_objects.
>
> Aside: Why is msm still using struct_mutex, it seems to be using
> dma_resv_lock for general buffer state protection?
>
> v2:
> - Add comment to explain why the ww ticket setup is separate (Rob)
> - Fix up error handling, we need to make sure we don't call
>   ww_acquire_fini without _init.
>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Rob Clark <robdclark at gmail.com>
> Cc: Sean Paul <sean at poorly.run>
> Cc: linux-arm-msm at vger.kernel.org
> Cc: freedreno at lists.freedesktop.org

found a few minutes to take this for a spin and seems fine.. t-b && r-b

BR,
-R

> ---
>  drivers/gpu/drm/msm/msm_gem_submit.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
> index fb1fdd7fa3ef..385d4965a8d0 100644
> --- a/drivers/gpu/drm/msm/msm_gem_submit.c
> +++ b/drivers/gpu/drm/msm/msm_gem_submit.c
> @@ -54,7 +54,6 @@ static struct msm_gem_submit *submit_create(struct drm_device *dev,
>
>         INIT_LIST_HEAD(&submit->node);
>         INIT_LIST_HEAD(&submit->bo_list);
> -       ww_acquire_init(&submit->ticket, &reservation_ww_class);
>
>         return submit;
>  }
> @@ -390,8 +389,6 @@ static void submit_cleanup(struct msm_gem_submit *submit)
>                 list_del_init(&msm_obj->submit_entry);
>                 drm_gem_object_put(&msm_obj->base);
>         }
> -
> -       ww_acquire_fini(&submit->ticket);
>  }
>
>  int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
> @@ -408,6 +405,7 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
>         struct msm_ringbuffer *ring;
>         int out_fence_fd = -1;
>         struct pid *pid = get_pid(task_pid(current));
> +       bool has_ww_ticket = false;
>         unsigned i;
>         int ret, submitid;
>         if (!gpu)
> @@ -489,6 +487,9 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
>         if (ret)
>                 goto out;
>
> +       /* copy_*_user while holding a ww ticket upsets lockdep */
> +       ww_acquire_init(&submit->ticket, &reservation_ww_class);
> +       has_ww_ticket = true;
>         ret = submit_lock_objects(submit);
>         if (ret)
>                 goto out;
> @@ -588,6 +589,8 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
>
>  out:
>         submit_cleanup(submit);
> +       if (has_ww_ticket)
> +               ww_acquire_fini(&submit->ticket);
>         if (ret)
>                 msm_gem_submit_free(submit);
>  out_unlock:
> --
> 2.24.0
>