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

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Thu, Mar 05, 2026 at 12:35:27PM +0530, Maninder Singh 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)

Looking at x86, it also has THREAD_INFO_IN_TASK, but I see nothing like
this in show_stack(). How come x86 isn't similarly buggy?

> 
> 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
> ...

"otherwise if someone calls show_stack() for task" ... and the stack
trace given stops at show_stack() and doesn't show the "someone".

I'd like to know _how_ this happens, and why ARM64 and now 32-bit ARM
are different from x86.

-- 
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