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SubjectRe: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context
On 12/11, Srivatsa S. Bhat wrote:
>
> IOW, the hotplug readers just increment/decrement their per-cpu refcounts
> when no writer is active.

plus cli/sti ;) and increment/decrement are atomic.

At first glance looks correct to me, but I'll try to read it carefully
later.

A couple of minor nits,

> +static DEFINE_PER_CPU(bool, writer_signal);

Why it needs to be per-cpu? It can be global and __read_mostly to avoid
the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
into a single cacheline...

> +void get_online_cpus_atomic(void)
> +{
> + unsigned long flags;
> +
> + preempt_disable();
> +
> + if (cpu_hotplug.active_writer == current)
> + return;
> +
> + local_irq_save(flags);

Yes... this is still needed, we are going to increment reader_percpu_refcnt
unconditionally and this makes reader_nested_percpu() == T.

But,

> +void put_online_cpus_atomic(void)
> +{
> + unsigned long flags;
> +
> + if (cpu_hotplug.active_writer == current)
> + goto out;
> +
> + local_irq_save(flags);
> +
> + /*
> + * We never allow heterogeneous nesting of readers. So it is trivial
> + * to find out the kind of reader we are, and undo the operation
> + * done by our corresponding get_online_cpus_atomic().
> + */
> + if (__this_cpu_read(reader_percpu_refcnt))
> + __this_cpu_dec(reader_percpu_refcnt);
> + else
> + read_unlock(&hotplug_rwlock);
> +
> + local_irq_restore(flags);
> +out:
> + preempt_enable();
> +}

Do we really need local_irq_save/restore in put_ ?

Oleg.



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