Re: [PATCH 8/8] drm/panfrost: Make sure the shrinker does not reclaim referenced BOs

[Robin Murphy]

On 29/11/2019 1:59 pm, Boris Brezillon wrote:
Userspace might tag a BO purgeable while it's still referenced by GPU
jobs. We need to make sure the shrinker does not purge such BOs until
all jobs referencing it are finished.

Nit: for extra robustness, perhaps it's worth using the refcount_t API 
rather than bare atomic_t?

Robin.

Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
Cc: <sta...@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezil...@collabora.com>
---
  drivers/gpu/drm/panfrost/panfrost_drv.c          | 1 +
  drivers/gpu/drm/panfrost/panfrost_gem.h          | 6 ++++++
  drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c | 2 ++
  drivers/gpu/drm/panfrost/panfrost_job.c          | 7 ++++++-
  4 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c 
b/drivers/gpu/drm/panfrost/panfrost_drv.c
index b406b5243b40..297c0e7304d2 100644
--- a/drivers/gpu/drm/panfrost/panfrost_drv.c
+++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
@@ -166,6 +166,7 @@ panfrost_lookup_bos(struct drm_device *dev,
                        break;
                }
  
+		atomic_inc(&bo->gpu_usecount);
                job->mappings[i] = mapping;
        }
  
diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.h b/drivers/gpu/drm/panfrost/panfrost_gem.h
index ca1bc9019600..b3517ff9630c 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem.h
+++ b/drivers/gpu/drm/panfrost/panfrost_gem.h
@@ -30,6 +30,12 @@ struct panfrost_gem_object {
                struct mutex lock;
        } mappings;
  
+	/*
+        * Count the number of jobs referencing this BO so we don't let the
+        * shrinker reclaim this object prematurely.
+        */
+       atomic_t gpu_usecount;
+
        bool noexec             :1;
        bool is_heap            :1;
  };
diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c 
b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
index b36df326c860..288e46c40673 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
+++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
@@ -41,6 +41,8 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
        struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
        struct panfrost_gem_object *bo = to_panfrost_bo(obj);
  
+	if (atomic_read(&bo->gpu_usecount))
+               return false;
  
  	if (!mutex_trylock(&shmem->pages_lock))
                return false;
diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c 
b/drivers/gpu/drm/panfrost/panfrost_job.c
index c85d45be3b5e..2b12aa87ff32 100644
--- a/drivers/gpu/drm/panfrost/panfrost_job.c
+++ b/drivers/gpu/drm/panfrost/panfrost_job.c
@@ -270,8 +270,13 @@ static void panfrost_job_cleanup(struct kref *ref)
        dma_fence_put(job->render_done_fence);
  
  	if (job->mappings) {
-               for (i = 0; i < job->bo_count; i++)
+               for (i = 0; i < job->bo_count; i++) {
+                       if (!job->mappings[i])
+                               break;
+
+                       atomic_dec(&job->mappings[i]->obj->gpu_usecount);
                        panfrost_gem_mapping_put(job->mappings[i]);
+               }
                kvfree(job->mappings);
        }
  

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