Re: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8)

[Srikar Dronamraju]

Hi Mathieu,

> +static int do_cpu_opv(struct cpu_op *cpuop, int cpuopcnt,
> +		      struct cpu_opv_vaddr *vaddr_ptrs, int cpu)
> +{
> +	struct mm_struct *mm = current->mm;
> +	int ret;
> +
> +retry:
> +	if (cpu != raw_smp_processor_id()) {
> +		ret = push_task_to_cpu(current, cpu);
> +		if (ret)
> +			goto check_online;
> +	}
> +	down_read(&mm->mmap_sem);
> +	ret = vaddr_ptrs_check(vaddr_ptrs);
> +	if (ret)
> +		goto end;
> +	preempt_disable();
> +	if (cpu != smp_processor_id()) {
> +		preempt_enable();
> +		up_read(&mm->mmap_sem);
> +		goto retry;
> +	}

If we have a higher priority task/s either pinned to the cpu, dont we end up
in busy-looping till the task exits/sleeps?

> +	ret = __do_cpu_opv(cpuop, cpuopcnt);
> +	preempt_enable();
> +end:
> +	up_read(&mm->mmap_sem);
> +	return ret;
> +
> +check_online:
> +	/*
> +	 * push_task_to_cpu() returns -EINVAL if the requested cpu is not part
> +	 * of the current thread's cpus_allowed mask.
> +	 */
> +	if (ret == -EINVAL)
> +		return ret;
> +	get_online_cpus();
> +	if (cpu_online(cpu)) {
> +		put_online_cpus();
> +		goto retry;
> +	}