Linux – What Does It Mean When Using Linux?

The explicit goal of the GNU project is to provide a complete open source/libre/free operating system.

Are there any GNU distributions which use only these packages -- i.e. a "pure" GNU operating system that runs on only GNU packages?

There is a reference here to an official sounding GNU binary distro based on Hurd which "consists of GNU Mach, the Hurd, the C library and many applications". It may or may not be currently maintained, however, as I couldn't find any other online references to it. But it does sound like it fits your criteria.

I'm not particularly interested on whether this would be a practical operating system, just if it's theoretically possible to run GNU Hurd with purely the GNU packages.

The answer to the previous question implies an obvious answer WRT Hurd. Of course, it might help to define more precisely what would count as a reasonably complete "operating system". I'll provide two definitions:

1. A collection of software sufficient to boot up to a shell prompt.

2. A system which fulfills POSIX criteria. This is essentially a stricter version of #1, since the highest level mandatory entity in a POSIX system would be the shell.

This is a little arbitrary, since an operating system designed to fulfill some special purpose might not need a shell at all. However, in that case it would become a more specific question about the nature of the "special purpose".

In any case, the answer is yes, although GNU's implementation of some things may not be 100% perfectly POSIX compliant (and there are a handful of required utilities, such as crontab, which GNU doesn't provide). Here are the potential components:

• Kernel (Hurd)
• C library (glibc)
• Essential utilities (GNU core-utils, etc.)
• Shell (bash, which is a GNU project)

I did not include a bootloader, since that is not part of the OS -- but in any case grub is also a GNU project.

I think this part of the clone(2) man page may clear up the difference re. the PID:

CLONE_THREAD (since Linux 2.4.0-test8)
If CLONE_THREAD is set, the child is placed in the same thread group as the calling process.
Thread groups were a feature added in Linux 2.4 to support the POSIX threads notion of a set of threads that share a single PID. Internally, this shared PID is the so-called thread group identifier (TGID) for the thread group. Since Linux 2.4, calls to getpid(2) return the TGID of the caller.

The "threads are implemented as processes" phrase refers to the issue of threads having had separate PIDs in the past. Basically, Linux originally didn't have threads within a process, just separate processes (with separate PIDs) that might have had some shared resources, like virtual memory or file descriptors. CLONE_THREAD and the separation of process ID^(*) and thread ID make the Linux behaviour look more like other systems and more like the POSIX requirements in this sense. Though technically the OS still doesn't have separate implementations for threads and processes.

Signal handling was another problematic area with the old implementation, this is described in more detail in the paper @FooF refers to in their answer.

As noted in the comments, Linux 2.4 was also released in 2001, the same year as the book, so it's not surprising the news didn't get to that print.