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Date:	Wed, 19 Dec 2012 20:14:36 +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 12/19, Srivatsa S. Bhat wrote:
>
> BTW, there is a small problem with the synchronize_sched() approach:
> We can't generalize the synchronization scheme as a generic percpu rwlock
> because the writer's role is split into 2, the first part (the one having
> synchronize_sched()) which should be run in process context, and the
> second part (the rest of it), which can be run in atomic context.

Yes,

> That is, needing the writer to be able to sleep (due to synchronize_sched())
> will make the locking scheme unusable (in other usecases) I think. And
> the split (blocking and non-blocking part) itself seems hard to express
> as a single writer API.

I do not think this is the problem.

We need 2 helpers for writer, the 1st one does synchronize_sched() and the
2nd one takes rwlock. A generic percpu_write_lock() simply calls them both.

In fact I think that the 1st helper should not do synchronize_sched(),
and (say) cpu_hotplug_begin() should call it itself. Because if we also
change cpu_hotplug_begin() to use percpu_rw_sem we do not want to do
synchronize_sched() twice. But lets ignore this for now.

But,

> Hmmm.. so what do we do? Shall we say "We anyway have to do smp_rmb() in the
> reader in the fast path. Instead let's do a full smp_mb() and get rid of
> the synchronize_sched() in the writer, and thereby expose this synchronization
> scheme as generic percpu rwlocks?" Or should we rather use synchronize_sched()
> and keep this synchronization scheme restricted to CPU hotplug only?

Oh, I do not know ;)

To me, the main question is: can we use synchronize_sched() in cpu_down?
It is slow.

Oleg.

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