+ scripts-gdb-fix-kgdb-probing-on-single-core-systems.patch added to mm-nonmm-unstable branch

+ scripts-gdb-fix-kgdb-probing-on-single-core-systems.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: scripts/gdb: fix kgdb probing on single-core systems
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     scripts-gdb-fix-kgdb-probing-on-single-core-systems.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/scripts-gdb-fix-kgdb-probing-on-single-core-systems.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Illia Ostapyshyn <illia@yshyn.com>
Subject: scripts/gdb: fix kgdb probing on single-core systems
Date: Sat, 3 May 2025 14:32:31 +0200

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and update an outdated example of its usage.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>
---

 scripts/gdb/linux/utils.py |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/scripts/gdb/linux/utils.py~scripts-gdb-fix-kgdb-probing-on-single-core-systems
+++ a/scripts/gdb/linux/utils.py
@@ -200,7 +200,7 @@ def get_gdbserver_type():
 
     def probe_kgdb():
         try:
-            thread_info = gdb.execute("info thread 2", to_string=True)
+            thread_info = gdb.execute("info thread 1", to_string=True)
             return "shadowCPU" in thread_info
         except gdb.error:
             return False
_

Patches currently in -mm which might be from illia@yshyn.com are

scripts-gdb-fix-kgdb-probing-on-single-core-systems.patch
scripts-gdb-update-documentation-for-lx_per_cpu.patch