Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
> Add a new system call to allow applications to block in the kernel and
> wait for user interrupts.
>
> <The current implementation doesn't support waking up from other
> blocking system calls like sleep(), read(), epoll(), etc.
>
> uintr_wait() is a placeholder syscall while we decide on that
> behaviour.>
>
> When the application makes this syscall the notification vector is
> switched to a new kernel vector. Any new SENDUIPI will invoke the kernel
> interrupt which is then used to wake up the process.
>
> Currently, the task wait list is global one. To make the implementation
> scalable there is a need to move to a distributed per-cpu wait list.

How are per cpu wait lists going to solve the problem?

> +
> +/*
> + * Handler for UINTR_KERNEL_VECTOR.
> + */
> +DEFINE_IDTENTRY_SYSVEC(sysvec_uintr_kernel_notification)
> +{
> +	/* TODO: Add entry-exit tracepoints */
> +	ack_APIC_irq();
> +	inc_irq_stat(uintr_kernel_notifications);
> +
> +	uintr_wake_up_process();

So this interrupt happens for any of those notifications. How are they
differentiated? 
>  
> +int uintr_receiver_wait(void)
> +{
> +	struct uintr_upid_ctx *upid_ctx;
> +	unsigned long flags;
> +
> +	if (!is_uintr_receiver(current))
> +		return -EOPNOTSUPP;
> +
> +	upid_ctx = current->thread.ui_recv->upid_ctx;
> +	upid_ctx->upid->nc.nv = UINTR_KERNEL_VECTOR;
> +	upid_ctx->waiting = true;
> +	spin_lock_irqsave(&uintr_wait_lock, flags);
> +	list_add(&upid_ctx->node, &uintr_wait_list);
> +	spin_unlock_irqrestore(&uintr_wait_lock, flags);
> +
> +	set_current_state(TASK_INTERRUPTIBLE);

Because we have not enough properly implemented wait primitives you need
to open code one which is blantantly wrong vs. a concurrent wake up?

> +	schedule();

How is that correct vs. a spurious wakeup? What takes care that the
entry is removed from the list?

Again. We have proper wait primitives.

> +	return -EINTR;
> +}
> +
> +/*
> + * Runs in interrupt context.
> + * Scan through all UPIDs to check if any interrupt is on going.
> + */
> +void uintr_wake_up_process(void)
> +{
> +	struct uintr_upid_ctx *upid_ctx, *tmp;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&uintr_wait_lock, flags);
> +	list_for_each_entry_safe(upid_ctx, tmp, &uintr_wait_list, node) {
> +		if (test_bit(UPID_ON, (unsigned long*)&upid_ctx->upid->nc.status)) {
> +			set_bit(UPID_SN, (unsigned long *)&upid_ctx->upid->nc.status);
> +			upid_ctx->upid->nc.nv = UINTR_NOTIFICATION_VECTOR;
> +			upid_ctx->waiting = false;
> +			wake_up_process(upid_ctx->task);
> +			list_del(&upid_ctx->node);

So any of these notification interrupts does a global mass wake up? How
does that make sense?

> +		}
> +	}
> +	spin_unlock_irqrestore(&uintr_wait_lock, flags);
> +}
> +
> +/* Called when task is unregistering/exiting */
> +static void uintr_remove_task_wait(struct task_struct *task)
> +{
> +	struct uintr_upid_ctx *upid_ctx, *tmp;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&uintr_wait_lock, flags);
> +	list_for_each_entry_safe(upid_ctx, tmp, &uintr_wait_list, node) {
> +		if (upid_ctx->task == task) {
> +			pr_debug("wait: Removing task %d from wait\n",
> +				 upid_ctx->task->pid);
> +			upid_ctx->upid->nc.nv = UINTR_NOTIFICATION_VECTOR;
> +			upid_ctx->waiting = false;
> +			list_del(&upid_ctx->node);
> +		}

What? You have to do a global list walk to find the entry which you
added yourself?

Thanks,

        tglx