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SubjectRe: [PATCH] ARM64: fixed dump_backtrace() when task running on another cpu
Hi,

On Thu, Apr 09, 2020 at 05:38:16PM +0800, Wang Qing wrote:
> We cannot get FP and PC when the task is running on another CPU,
> task->thread.cpu_context is the last time the task was switched out,
> it's better to give a reminder than to provide wrong information.
>
> Signed-off-by: Wang Qing <wangqing@vivo.com>

Are you seeing this happen anywhere in particular today?

> ---
> arch/arm64/kernel/traps.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index cf402be..c04e3e8 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -106,6 +106,14 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
> start_backtrace(&frame,
> (unsigned long)__builtin_frame_address(0),
> (unsigned long)dump_backtrace);
> + } else if (tsk->on_cpu) {
> + /*
> + * The task is running in another cpu, so the call stack
> + * is changing and we cannot get it.
> + */
> + pr_warn("tsk: %s is running in CPU%d, Don't call trace!\n",
> + tsk->comm, tsk->cpu);

I believe that we can race with a concurrent write to tsk->cpu in both
cases above. We could use READ_ONCE() to get a snapshot, but we can
still race and miss cases where the task was runnning as we backtrace
it.

Thanks,
Mark.

> + return;
> } else {
> /*
> * task blocked in __switch_to
> --
> 2.7.4
>

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