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Date: Wed, 12 Dec 2012 18:17:20 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc: tglx@...utronix.de, peterz@...radead.org,
paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [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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