Are Linux kernel threads really kernel processes?

The documentation can be pretty confusing, so here is the "real" Linux model:

• inside the Linux kernel, something that can be run (& scheduled) is called a "process",
• each process has a system-unique Process ID (PID), and a Thread Group ID (TGID),
• a "normal" process has PID=TGID and no other process shares this TGID value,
• a "threaded" process is a process which TGID value is shared by other processes,
• several processes sharing the same TGID also share, at least, the same memory space and signal handlers (sometimes more),
• if a "threaded" process has PID=TGID, it can be called "the main thread",
• calling getpid() from any process will return its TGID (= "main thread" PID),
• calling gettid() from any process will return its PID (!),
• any kind of process can be created with the clone(2) system call,
• what is shared between processes is decided by passing specific flags to clone(2),
• folders' numeric names you can list with ls /proc as /proc/NUMBER are TGIDs,
• folders' numeric names in /proc/TGID/task as /proc/TGID/task/NUMBER are PIDs,
• even though you don't see every existing PIDs with ls /proc, you can still do cd /proc/any_PID.

Conclusion: from the kernel point of view, only processes exist, each having their own unique PID, and a so-called thread is just a different kind of process (sharing, at least, the same memory space and signal handlers with one or several other-s).

Note: the implementation of the "thread" concept in Linux has led to a vocabulary confusion, and if getpid() is lying to you does not do what you thought, it is because its behavior follows POSIX compatibility (threads are supposed to share a common PID).

There is absolutely no difference between a thread and a process on Linux. If you look at clone(2) you will see a set of flags that determine what is shared, and what is not shared, between the threads.

Classic processes are just threads that share nothing; you can share what components you want under Linux.

This is not the case on other OS implementations, where there are much more substantial differences.

Threads are processes under Linux. They are created with the clone system call, which returns a process ID that can be sent a signal via the kill system call, just like a process. Thread processes are visible in ps output. The clone call is passed flags which determine how much of the parent process's environment is shared with the thread process.