Re: [PATCH v12 06/14] unwind_user/deferred: Add deferred unwinding interface

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 2 Jul 2025 15:36:00 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> union unwind_task_id {
> 	struct {
> 		u32		task_id;
> 		u32		cnt;
> 	}
> 	u64 id;
> };
> 
> static u64 get_cookie(struct unwind_task_info *info)
> {
> 	u32 cnt = READ_ONCE(info->id.cnt);
> 	u32 new_cnt;
> 
> 	if (cnt & 1)
> 		return info->id;
> 
> 	if (unlikely(!info->id.task_id)) {
> 		u32 task_id = local_clock();
> 
> 		cnt = 0;
> 		if (try_cmpxchg(&info->id.task_id, &cnt, task_id))
> 			task_id = cnt;
> 	}
> 
> 	new_cnt = cnt + 3;
> 	if (try_cmpxchg(&info->id, &cnt, new_cnt))
> 		new_cnt = cnt; // try_cmpxchg() expects something
> 
> 	return info->id;
> }

Honestly I think this is way overkill. What I would do, is to have the
cookie saved in the event be 64 bit, but we can start with the
simple 32 bit solution keeping the top 32 bits zeros. If this does
indeed become an issue in the future, we could fix it with a 64 bit
number. By making sure all the exposed "cookies" are 64 bit, it should
not break anything. The cookie is just supposed to be a random unique
number that associates a request with its deferred user space stack
trace.

With any exposed cookies to user space being 64 bits, this should not
be an issue to address in the future.

-- Steve