From 6be7045c7756caf2d445dc66473e45ac5779cd55 Mon Sep 17 00:00:00 2001 From: Illia Ostapyshyn Date: Sat, 3 May 2025 14:32:31 +0200 Subject: 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featuring a single CPU and (2) update the documentation to reflect the current usage of lx_per_cpu() and update an outdated example of its usage.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 Cc: Alex Shi Cc: Brendan Jackman Cc: Dongliang Mu Cc: Florian Rommel Cc: Hu Haowen <2023002089@link.tyut.edu.cn> Cc: Jan Kiszka Cc: Jonathan Corbet Cc: Kieran Bingham Cc: Yanteng Si Signed-off-by: Andrew Morton --- scripts/gdb/linux/utils.py | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/scripts/gdb/linux/utils.py b/scripts/gdb/linux/utils.py index 03ebdccf5f69..877404e92dbb 100644 --- a/scripts/gdb/linux/utils.py +++ b/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 -- cgit v1.2.3-59-g8ed1b