On June 19, 2025 4:57:17 AM EDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>On Tue, Jun 10, 2025 at 08:54:28PM -0400, Steven Rostedt wrote:
>
>>
>> + info->nmi_timestamp = local_clock();
>> + *timestamp = info->nmi_timestamp;
>> + inited_timestamp = true;
>> + }
>> +
>> + if (info->pending)
>> + return 1;
>> +
>> + ret = task_work_add(current, &info->work, TWA_NMI_CURRENT);
>> + if (ret < 0) {
>> + /*
>> + * If this set nmi_timestamp and is not using it,
>> + * there's no guarantee that it will be used.
>> + * Set it back to zero.
>> + */
>> + if (inited_timestamp)
>> + info->nmi_timestamp = 0;
>> + return ret;
>> + }
>> +
>> + info->pending = 1;
>> +
>> + return 0;
>> +}
>> +
>> /**
>> * unwind_deferred_request - Request a user stacktrace on task exit
>> * @work: Unwind descriptor requesting the trace
>> @@ -139,31 +207,38 @@ static void unwind_deferred_task_work(struct callback_head *head)
>> int unwind_deferred_request(struct unwind_work *work, u64 *timestamp)
>> {
>> struct unwind_task_info *info = ¤t->unwind_info;
>> + int pending;
>> int ret;
>>
>> *timestamp = 0;
>>
>> if ((current->flags & (PF_KTHREAD | PF_EXITING)) ||
>> !user_mode(task_pt_regs(current)))
>> return -EINVAL;
>>
>> + if (in_nmi())
>> + return unwind_deferred_request_nmi(work, timestamp);
>
>So nested NMI is a thing -- AFAICT this is broken in the face of nested
>NMI.
>
>Specifically, we mark all exceptions that can happen with IRQs disabled
>as NMI like (so that they don't go about taking locks etc.).
>
>So imagine you're in #DB, you're asking for an unwind, you do the above
>dance and get hit with NMI.
Does #DB make in_nmi() true? If that's the case then we do need to handle that.
-- Steve
>
>Then you get the NMI setting nmi_timestamp, and #DB overwriting it with
>a later value, and you're back up the creek without no paddles.
>
>
>Mix that with local_clock() that is only monotonic on a single CPU. And
>you ask for an unwind on CPU0, get migrated to CPU1 which for the
>argument will be behind, and see a timestamp 'far' in the future.
>
