Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

Peter Zijlstra
2022-01-21T11:48:38Z

On Thu, Jan 20, 2022 at 04:55:22PM +0100, Peter Zijlstra wrote:

> +SYSCALL_DEFINE2(umcg_wait, u32, flags, u64, timo)
> +{
> + struct task_struct *tsk = current;
> + struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
> + bool worker = tsk->flags & PF_UMCG_WORKER;
> + int ret;
> +
> + if (!self || flags)
> + return -EINVAL;
> +
> + if (worker) {
> + tsk->flags &= ~PF_UMCG_WORKER;
> + if (timo)
> + return -ERANGE;
> + }
> +
> + /* see umcg_sys_{enter,exit}() syscall exceptions */
> + ret = umcg_pin_pages();
> + if (ret)
> + goto unblock;
> +
> + /*
> + * Clear UMCG_TF_COND_WAIT *and* check state == RUNNABLE.
> + */
> + ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNABLE);
> + if (ret)
> + goto unpin;
> +
> + ret = umcg_wake_next(tsk, self);
> + if (ret)
> + goto unpin;
> +
> + if (worker) {
> + /*
> + * If this fails it is possible ::next_tid is already running
> + * while this task is not going to block. This violates our
> + * constraints.
> + *
> + * That said, pretty much the only way to make this fail is by
> + * force munmap()'ing things. In which case one is most welcome
> + * to the pieces.
> + */
> + ret = umcg_enqueue_and_wake(tsk);
> + if (ret)
> + goto unpin;
> + }
> +
> + umcg_unpin_pages();
> +
> + ret = umcg_wait(timo);
> + switch (ret) {
> + case 0: /* all done */
> + case -EINTR: /* umcg_notify_resume() will continue the wait */

So I was playing with the whole worker timeout thing last night and
realized this is broken. If we get a signal while we have a timeout, the
timeout gets lost.

I think the easiest solution is to have umcg_notify_resume() also resume
the timeout, but the first pass of that was yuck, so I need to try
again.

Related, by moving the whole enqueue-and-wake thing into the timeout, we
get more 'fun' failure cases :-(

Oh well..

> + ret = 0;
> + break;
> +
> + default:
> + goto unblock;
> + }
> +out:
> + if (worker)
> + tsk->flags |= PF_UMCG_WORKER;
> + return ret;
> +
> +unpin:
> + umcg_unpin_pages();
> +unblock:
> + umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNING);
> + goto out;
> +}