By the way this is how threads work on Linux to this day. LinuxThreads were exactly this and NPTL is mostly about doing magic with RT signals and shadowing libc

By the way this is how threads work on Linux to this day. LinuxThreads were exactly this and NPTL is mostly about doing magic with RT signals and shadowing libc symbols such that stuff that changes global process state works reliably. Along the way the kernel got some awareness of userspace playing such tricks and got futex(), but still it is mostly implemented as bunch of userspace magic that makes essentially unrelated processes look like threads of same process. One nice consequence of this is that all the PTHREAD_WHATEVER_PSHARED stuff simply works (in contrast to BSD derivates, where you have to choose whether you want to use IPC primitive across threads or across processed and when you want both you get -ENOSYS, -EINVAL or somewhat hilariously -ENOMEM with manpage containing rationale along the lines “POSIX says that this has to be possible and that there can be global limit on number of such objects. It does not say that the limit should be larger than zero, so this always fails with -ENOMEM”)