linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Date:	Wed, 19 Dec 2012 17:39:00 +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

(sorry if you see this email twice)

On 12/19, Srivatsa S. Bhat wrote:
>
> On 12/19/2012 01:13 AM, Oleg Nesterov wrote:
>
> >> I tried thinking about other ways to avoid that smp_mb() in the reader,
> >
> > Just in case, I think there is no way to avoid mb() in _get (although
> > perhaps it can be "implicit").
> >
>
> Actually, I was trying to avoid mb() in the _fastpath_, when there is no
> active writer. I missed stating that clearly, sorry.

Yes, I meant the fastpath too,

> > The writer changes cpu_online_mask and drops the lock. We need to ensure
> > that the reader sees the change in cpu_online_mask after _get returns.
> >
>
> The write_unlock() will ensure the completion of the update to cpu_online_mask,
> using the same semi-permeable logic that you pointed above. So readers will
> see the update as soon as the writer releases the lock, right?

Why?

Once again, the writer (say) removes CPU_1 from cpu_online_mask, and
sets writer_signal[0] = 0, this "enables" fast path on CPU_0.

But, without a barrier, there is no guarantee that CPU_0 will see the
change in cpu_online_mask after get_online_cpus_atomic() checks
writer_signal[0] and returns.

Hmm. And this means that the last version lacks the additional rmb()
(at least) if writer_active() == F.

Oleg.

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