Re: [PATCH v5 3/9] unwind deferred: Use bitmask to determine which callbacks to call

Mathieu Desnoyers
2025-04-28T18:13:24Z

On 2025-04-28 14:12, Steven Rostedt wrote:
> On Mon, 28 Apr 2025 14:00:07 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>
>>>>
>>>> It is enough to guard with RCU ? See syscall_regfunc() from
>>>> tracepoint.c where we do:
>>>>
>>>> read_lock(&tasklist_lock);
>>>> for_each_process_thread(p, t) {
>>>> set_task_syscall_work(t, SYSCALL_TRACEPOINT);
>>>> }
>>>> read_unlock(&tasklist_lock);
>>>>
>>>> To prevent concurrent fork from adding threads while we
>>>> iterate, thus opening the possibility of missing a clear
>>>> due to a concurrent fork + set bit.
>>>
>>> A set_bit only would happen if the callback was live and accepting new
>>> callback requests. It's a bug for a tracer to call unwind_deferred_cancel()
>>> and then call unwind_deferred_request() (which would set the bit). We could
>>> possibly set the tracer's unwind descriptor id to -1, and do an
>>> WARN_ON_ONCE() in unwind_deferred_request() if the tracer's id is negative.
>>>
>>> The loop is called under the callback_mutex, where no new tracer could
>>> register and be assigned that bit.
>>
>> Ah, that's the piece I missed. The callback_mutex prevents reallocation
>> of the ID by unwind_deferred_init while iterating on the tasks.
>>
>> One more comment: if we change the linked list for an array (or make the
>> linked list an RCU list), can we remove the callback_mutex from
>> unwind_deferred_task_work by turning it into an RCU read-side ?
>>
>> Then we just need to wait for a grace period before returning from
>> unwind_deferred_cancel, which then allows the caller to reclaim "work".
>>
>> Taking the callback_mutex in unwind_deferred_task_work will end up being
>> the single thing that does a lot of cache line bouncing across CPUs when
>> hit heavily by tracers.
>
> I'm not against this, but again, that's an optimization. I want to keep the
> initial code simple. And then add the more complex optimizations when this
> is stable.

That works for me,

Thanks!

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.