[PATCH 5.10 664/798] m68k: Fix backtraces for non-running tasks

5.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Karl Mehltretter <redacted>

[ Upstream commit 09f71dbeb287e367beea56d9ac8a3d356ef5e02a ]

When no stack pointer is supplied, show_stack() starts at thread.esp0.
This is the saved userspace register frame, not where a task was switched
out. For kernel threads it points to the zeroed frame created by
copy_thread(), so sched_show_task() prints no useful backtrace.

Use thread.ksp for non-current tasks. It is saved by switch_to() and is
already used by __get_wchan(). For current, start at the current stack
frame, as is already done when no task is supplied.

Tested on qemu-system-m68k -M virt using SysRq-t. All task dumps contained
resolved frames.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Karl Mehltretter <redacted>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://patch.msgid.link/20260723041055.47289-1-kmehltretter@gmail.com (local)
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/m68k/kernel/traps.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index f6706ef6b195e..245690ac613ef 100644
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c

@@ -941,8 +941,8 @@ void show_stack(struct task_struct *task, unsigned long *stack,
 	int i;
 
 	if (!stack) {
-		if (task)
-			stack = (unsigned long *)task->thread.esp0;
+		if (task && task != current)
+			stack = (unsigned long *)task->thread.ksp;
 		else
 			stack = (unsigned long *)&stack;
 	}

-- 
2.53.0