x86/process: setup io_threads more like normal user space threads

As io_threads are fully set up USER threads it's clearer to separate the
code path from the KTHREAD logic.

The only remaining difference to user space threads is that io_threads
never return to user space again. Instead they loop within the given
worker function.

The fact that they never return to user space means they don't have an
user space thread stack. In order to indicate that to tools like gdb we
reset the stack and instruction pointers to 0.

This allows gdb attach to user space processes using io-uring, which like
means that they have io_threads, without printing worrying message like
this:

  warning: Selected architecture i386:x86-64 is not compatible with reported target architecture i386

  warning: Architecture rejected target-supplied description

The output will be something like this:

  (gdb) info threads
    Id   Target Id                  Frame
  * 1    LWP 4863 "io_uring-cp-for" syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
    2    LWP 4864 "iou-mgr-4863"    0x0000000000000000 in ?? ()
    3    LWP 4865 "iou-wrk-4863"    0x0000000000000000 in ?? ()
  (gdb) thread 3
  [Switching to thread 3 (LWP 4865)]
  #0  0x0000000000000000 in ?? ()
  (gdb) bt
  #0  0x0000000000000000 in ?? ()
  Backtrace stopped: Cannot access memory at address 0x0

Fixes: 4727dc20 ("arch: setup PF_IO_WORKER threads like PF_KTHREAD")
Link: https://lore.kernel.org/io-uring/044d0bad-6888-a211-e1d3-159a4aeed52d@polymtl.ca/T/#m1bbf5727e3d4e839603f6ec7ed79c7eebfba6267


Signed-off-by: Stefan Metzmacher <metze@samba.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Andy Lutomirski <luto@kernel.org>
cc: linux-kernel@vger.kernel.org
cc: io-uring@vger.kernel.org
cc: x86@kernel.org
Link: https://lore.kernel.org/r/20210505110310.237537-1-metze@samba.org


Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>