Re: [PATCH 1/1] arm: get task_stack reference before dump_backtrace

On Thu, Mar 5, 2026 at 8:05 AM Maninder Singh [off-list ref] wrote:

With Support of THREAD_INFO_IN_TASK, stack of task can be
freed earlier than task (even if task's reference is taken),
and it needs separate reference with try_get_task_stack()
before using the stack.
Otherwise if someone calls show_stack() for task, it can oops
the kernel like below: (Tried with normal race of show_stack when
task still exists, but its stack is freed)

8<--- cut here ---
Unable to handle kernel paging request at virtual address f8aebec4 when read
[f8aebec4] *pgd=83c2c811, *pte=00000000, *ppte=00000000
Internal error: Oops: 7 [#1] SMP ARM
..
CPU: 0 UID: 0 PID: 70 Comm: cat Not tainted 7.0.0-rc2-next-20260302+ #26 VOLUNTARY
..
PC is at __read_once_word_nocheck+0x0/0x8
LR is at unwind_frame+0x6b0/0xa90
...
Call trace:
 __read_once_word_nocheck from unwind_frame+0x6b0/0xa90
 unwind_frame from unwind_backtrace+0x178/0x1e0
 unwind_backtrace from show_stack+0x10/0x14
...

ARM64 also takes care of it in dump_backtrace(), so same logic
is added for ARM also.

Fixes: 18ed1c01a7dd ("ARM: smp: Enable THREAD_INFO_IN_TASK")
Signed-off-by: Maninder Singh <redacted>

Ard B should review this but it looks right to me:
Acked-by: Linus Walleij <linusw@kernel.org>

Yours,
Linus Walleij