drm/i915: Remove locking from i915_gem_object_prepare_read/write

Execbuffer submission will perform its own WW locking, and we
cannot rely on the implicit lock there.

This also makes it clear that the GVT code will get a lockdep splat when
multiple batchbuffer shadows need to be performed in the same instance,
fix that up.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200819140904.1708856-7-maarten.lankhorst@linux.intel.com
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -335,12 +335,20 @@ i915_gem_shmem_pread(struct drm_i915_gem_object *obj,
 	u64 remain;
 	int ret;
 
-	ret = i915_gem_object_prepare_read(obj, &needs_clflush);
+	ret = i915_gem_object_lock_interruptible(obj, NULL);
 	if (ret)
 		return ret;
 
+	ret = i915_gem_object_prepare_read(obj, &needs_clflush);
+	if (ret) {
+		i915_gem_object_unlock(obj);
+		return ret;
+	}
+
 	fence = i915_gem_object_lock_fence(obj);
 	i915_gem_object_finish_access(obj);
+	i915_gem_object_unlock(obj);
+
 	if (!fence)
 		return -ENOMEM;
 
@@ -734,12 +742,20 @@ i915_gem_shmem_pwrite(struct drm_i915_gem_object *obj,
 	u64 remain;
 	int ret;
 
-	ret = i915_gem_object_prepare_write(obj, &needs_clflush);
+	ret = i915_gem_object_lock_interruptible(obj, NULL);
 	if (ret)
 		return ret;
 
+	ret = i915_gem_object_prepare_write(obj, &needs_clflush);
+	if (ret) {
+		i915_gem_object_unlock(obj);
+		return ret;
+	}
+
 	fence = i915_gem_object_lock_fence(obj);
 	i915_gem_object_finish_access(obj);
+	i915_gem_object_unlock(obj);
+
 	if (!fence)
 		return -ENOMEM;