Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Oleg Nesterov]

Peter,

Sorry. Unlikely I will be able to read this patch today. So let me
ask another potentially wrong question without any thinking.

On 09/26, Peter Zijlstra wrote:
>
> +void __get_online_cpus(void)
> +{
> +again:
> +	/* See __srcu_read_lock() */
> +	__this_cpu_inc(__cpuhp_refcount);
> +	smp_mb(); /* A matches B, E */
> +	__this_cpu_inc(cpuhp_seq);
> +
> +	if (unlikely(__cpuhp_state == readers_block)) {

OK. Either we should see state = BLOCK or the writer should notice the
change in __cpuhp_refcount/seq. (altough I'd like to recheck this
cpuhp_seq logic ;)

> +		atomic_inc(&cpuhp_waitcount);
> +		__put_online_cpus();

OK, this does wake(cpuhp_writer).

>  void cpu_hotplug_begin(void)
>  {
> ...
> +	/*
> +	 * Notify new readers to block; up until now, and thus throughout the
> +	 * longish synchronize_sched() above, new readers could still come in.
> +	 */
> +	__cpuhp_state = readers_block;
> +
> +	smp_mb(); /* E matches A */
> +
> +	/*
> +	 * If they don't see our writer of readers_block to __cpuhp_state,
> +	 * then we are guaranteed to see their __cpuhp_refcount increment, and
> +	 * therefore will wait for them.
> +	 */
> +
> +	/* Wait for all now active readers to complete. */
> +	wait_event(cpuhp_writer, cpuhp_readers_active_check());

But. doesn't this mean that we need __wait_event() here as well?

Isn't it possible that the reader sees BLOCK but the writer does _not_
see the change in __cpuhp_refcount/cpuhp_seq? Those mb's guarantee
"either", not "both".

Don't we need to ensure that we can't check cpuhp_readers_active_check()
after wake(cpuhp_writer) was already called by the reader and before we
take the same lock?

Oleg.

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