drm/amdgpu: Use READ_ONCE() when reading the values in 'sdma_v4_4_2_ring_get_rptr'

Use READ_ONCE() instead of declaring the pointer volatile. To prevent
the compiler from refetching or reordering the read, so that the read
value is always consistent.

Link: https://lwn.net/Articles/624126/
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Guchun Chen <guchun.chen@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: Le Ma <le.ma@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/sdma_v4_4_2.c

@@ -154,13 +154,13 @@ static int sdma_v4_4_2_init_microcode(struct amdgpu_device *adev)
  */
 static uint64_t sdma_v4_4_2_ring_get_rptr(struct amdgpu_ring *ring)
 {
-	u64 *rptr;
+	u64 rptr;
 
 	/* XXX check if swapping is necessary on BE */
-	rptr = ((u64 *)&ring->adev->wb.wb[ring->rptr_offs]);
+	rptr = READ_ONCE(*((u64 *)&ring->adev->wb.wb[ring->rptr_offs]));
 
-	DRM_DEBUG("rptr before shift == 0x%016llx\n", *rptr);
-	return ((*rptr) >> 2);
+	DRM_DEBUG("rptr before shift == 0x%016llx\n", rptr);
+	return rptr >> 2;
 }
 
 /**