Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov <oleg@redhat.com>]

On 12/06, Srivatsa S. Bhat wrote:
>
> +void get_online_cpus_atomic(void)
> +{
> +	int c, old;
> +
> +	preempt_disable();
> +	read_lock(&hotplug_rwlock);

Confused... Why it also takes hotplug_rwlock?

> +
> +	for (;;) {
> +		c = atomic_read(&__get_cpu_var(atomic_reader_refcount));
> +		if (unlikely(writer_active(c))) {
> +			cpu_relax();
> +			continue;
> +		}
> +
> +		old = atomic_cmpxchg(&__get_cpu_var(atomic_reader_refcount),
> +				     c, c + 1);
> +
> +		if (likely(old == c))
> +			break;
> +
> +		c = old;
> +	}
> +}

	while (!atomic_inc_unless_negative(...))
		cpu_relax();
	
and atomic_dec_unless_positive() in disable_atomic_reader().


Obviously you can't use get_online_cpus_atomic() under rq->lock or
task->pi_lock or any other lock CPU_DYING can take. Probably this is
fine, but perhaps it makes sense to add the lockdep annotations.

Oleg.