If you are writing an operating system or a process switcher, then typically you use the __user_perproc_libspace() and __user_perthread_libspace() functions (which are always available) rather than re-implement __user_libspace().

If you have legacy source code that re-implements __user_libspace(), you do not have to change the re-implementation for single-threaded processes. However, you are likely to be required to do so for multi-threaded applications. For multi-threaded applications, use either or both of __user_perproc_libspace() and __user_perthread_libspace(), instead of __user_libspace().