[PATCH] drm/amdkfd: Fix a race condition of vram buffer unref in svm code

["Xiaogang.Chen" <xiaogang.chen@xxxxxxx>]

From: Xiaogang Chen <xiaogang.chen@xxxxxxx>

prange->svm_bo unref can happen in both mmu callback and a callback after
migrate to system ram. Both are async call in different tasks. Sync svm_bo
unref operation to avoid random "use-after-free".

Signed-off-by: Xiaogang.Chen <Xiaogang.Chen@xxxxxxx>
---
 drivers/gpu/drm/amd/amdkfd/kfd_svm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
index 70aa882636ab..8e246e848018 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
@@ -637,6 +637,15 @@ void svm_range_vram_node_free(struct svm_range *prange)
 {
 	svm_range_bo_unref(prange->svm_bo);
 	prange->ttm_res = NULL;
+	/* serialize prange->svm_bo unref */
+	mutex_lock(&prange->lock);
+	/* prange->svm_bo has not been unref */
+	if (prange->ttm_res) {
+		prange->ttm_res = NULL;
+		mutex_unlock(&prange->lock);
+		svm_range_bo_unref(prange->svm_bo);
+	} else
+		mutex_unlock(&prange->lock);
 }
 
 struct kfd_node *
-- 
2.25.1