Cycles: Fix race condition in attributes creation during SVM compilation

intern/cycles/render/svm.cpp

@@ -393,11 +393,13 @@ void SVMCompiler::add_node(const float4& f)
 
 uint SVMCompiler::attribute(ustring name)
 {
+	thread_scoped_spin_lock lock(attribute_lock_);
 	return shader_manager->get_attribute_id(name);
 }
 
 uint SVMCompiler::attribute(AttributeStandard std)
 {
+	thread_scoped_spin_lock lock(attribute_lock_);
 	return shader_manager->get_attribute_id(std);
 }
 

intern/cycles/render/svm.h

@@ -218,6 +218,8 @@ protected:
 	int max_stack_use;
 	uint mix_weight_offset;
 	bool compile_failed;
+
+	thread_spin_lock attribute_lock_;
 };
 
 CCL_NAMESPACE_END