Threads VS Processes in Linux

How is a process and a thread the same thing in Linux?

Linux didn't use to have special support for (POSIX) threads, and it simply treated them as processes that shared their address space as well as a few other resources (filedescriptors, signal actions, ...) with other "processes".

That implementation, while elegant, made certain things required for threads by POSIX difficult, so Linux did end up gaining that special support for threads and your premise is now no longer true.

Nevertheless, processes and threads still both remain represented as tasks within the kernel (but now the kernel has support for grouping those tasks into thread groups as well and APIs for working with those ((tgkill, tkill, exit_group, ...)).

You can google LinuxThreads and NPTL threads to learn more about the topic.

What is the difference between a process and a thread?

Both processes and threads are independent sequences of execution. The typical difference is that threads (of the same process) run in a shared memory space, while processes run in separate memory spaces.

I'm not sure what "hardware" vs "software" threads you might be referring to. Threads are an operating environment feature, rather than a CPU feature (though the CPU typically has operations that make threads efficient).

Erlang uses the term "process" because it does not expose a shared-memory multiprogramming model. Calling them "threads" would imply that they have shared memory.

Linux Kernel: Threading vs Process - task_struct vs thread_info

Threads in Linux are treated as processes that just happen to share some resources. Each thread has its own thread_info (at the bottom of the stack like you said) and its own task_struct. I can think of two reasons why they are maintained as separate structures.

1. thread_info is architecture dependent. task_struct is generic.
2. thread_info cuts into the size of the kernel stack for that process, so it should be kept small. thread_info is placed at the bottom of the stack as a micro-optimization that makes it possible to compute its address from the current stack pointer by rounding down by the stack size saving a CPU register.

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