Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Wed, Oct 30, 2024 at 10:47:55AM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 30, 2024 at 09:44:14AM -0400, Mathieu Desnoyers wrote:
> > What you want here is to move the point where you clear the task
> > cookie to _after_ completion of stack unwind. There are a few ways
> > this can be done:
> > 
> > A) Clear the task cookie in the task_work() right after the
> >    unwind_user_deferred() is completed. Downside: if some long task work
> >    happen to be done after the stack walk, a new unwind_user_deferred()
> >    could be issued again and we may end up looping forever taking stack
> >    unwind and never actually making forward progress.
> > 
> > B) Clear the task cookie after the exit_to_user_mode_loop is done,
> >    before returning to user-space, while interrupts are disabled.
> 
> Problem is, if another tracer calls unwind_user_deferred() for the first
> time, after the task work but before the task cookie gets cleared, it
> will see the cookie is non-zero and will fail to schedule another task
> work.  So its callback never gets called.

How about we:

  - clear task cookie on entry or exit from user space

  - use a different variable (rather than clearing of task cookie) to
    communicate whether task work is pending

  - keep track of which callbacks have been called for this cookie

something like so?

int unwind_user_deferred(struct unwind_callback *callback, u64 *ctx_cookie, void *data)
{
	struct unwind_task_info *info = &current->unwind_task_info;
	unsigned int work_pending = work_pending;
	u64 cookie = info->ctx_cookie;
	int idx = callback->idx;

	if (WARN_ON_ONCE(in_nmi()))
		return -EINVAL;

	if (!current->mm)
		return -EINVAL;

	guard(irqsave)();

	if (cookie && (info->pending_callbacks & (1 << idx))) {
		/* callback already scheduled */
		goto done;
	}

	/*
	 * If this is the first call from any caller since the most recent
	 * entry from user space, initialize the task context cookie.
	 */
	if (!cookie) {
		u64 cpu = raw_smp_processor_id();
		u64 ctx_ctr;

		__this_cpu_inc(unwind_ctx_ctr);
		ctx_ctr = __this_cpu_read(unwind_ctx_ctr);

		cookie = ctx_to_cookie(cpu, ctx_ctr);
		info->ctx_cookie = cookie;

	} else {
		if (cookie == info->last_cookies[idx]) {
			/*
			 * The callback has already been called with this unwind,
			 * nothing to do.
			 */
			goto done;
		}

		if (info->pending_callbacks & (1 << idx)) {
			/* callback already scheduled */
			goto done;
		}
	}

	info->pending_callbacks |= (1 << idx);
	info->privs[idx] = data;
	info->last_cookies[idx] = cookie;

	if (!info->task_pending) {
		info->task_pending = 1; /* cleared by unwind_user_task_work() */
		task_work_add(current, &info->work, TWA_RESUME);
	}

done:
	if (ctx_cookie)
		*ctx_cookie = cookie;
	return 0;
}

-- 
Josh