scripts/gdb: fix kgdb probing on single-core systems

scripts/gdb: fix kgdb probing on single-core systems

Patch series "scripts/gdb: Fixes related to lx_per_cpu()".

These patches (1) fix kgdb detection on systems featuring a single CPU and
(2) update the documentation to reflect the current usage of lx_per_cpu()
and update an outdated example of its usage.

This patch (of 2):

When requested the list of threads via qfThreadInfo, gdb_cmd_query in
kernel/debug/gdbstub.c first returns "shadow" threads for CPUs followed by
the actual tasks in the system.  Extended qThreadExtraInfo queries yield
"shadowCPU%d" as the name for the CPU core threads.

This behavior is used by get_gdbserver_type() to probe for KGDB by
matching the name for the thread 2 against "shadowCPU".  This breaks down
on single-core systems, where thread 2 is the first nonshadow thread.
Request the name for thread 1 instead.

As GDB assigns thread IDs in the order of their appearance, it is safe to
assume shadowCPU0 at ID 1 as long as CPU0 is not hotplugged.

Before:

(gdb) info threads
  Id   Target Id                      Frame
  1    Thread 4294967294 (shadowCPU0) kgdb_breakpoint ()
* 2    Thread 1 (swapper/0)           kgdb_breakpoint ()
  3    Thread 2 (kthreadd)            0x0000000000000000 in ?? ()
  ...
(gdb) p $lx_current().comm
Sorry, obtaining the current CPU is not yet supported with this gdb server.

After:

(gdb) info threads
  Id   Target Id                      Frame
  1    Thread 4294967294 (shadowCPU0) kgdb_breakpoint ()
* 2    Thread 1 (swapper/0)           kgdb_breakpoint ()
  3    Thread 2 (kthreadd)            0x0000000000000000 in ?? ()
  ...
(gdb) p $lx_current().comm
$1 = "swapper/0\000\000\000\000\000\000"

Link: https://lkml.kernel.org/r/20250503123234.2407184-1-illia@yshyn.com
Link: https://lkml.kernel.org/r/20250503123234.2407184-2-illia@yshyn.com
Signed-off-by: Illia Ostapyshyn <illia@yshyn.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Dongliang Mu <dzm91@hust.edu.cn>
Cc: Florian Rommel <mail@florommel.de>
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>