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Date:	Thu, 20 Dec 2012 14:42:03 +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 12/20, Srivatsa S. Bhat wrote:
>
> On 12/20/2012 12:44 AM, Oleg Nesterov wrote:
> >
> > 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.
> >
>
> Ah, that's the problem no? Users of reader-writer locks expect to run in
> atomic context (ie., they don't want to sleep).

Ah, I misunderstood.

Sure, percpu_write_lock() should be might_sleep(), and this is not
symmetric to percpu_read_lock().

> We can't expose an API that
> can make the task go to sleep under the covers!

Why? Just this should be documented. However I would not worry until we
find another user. Until then we do not even need to add percpu_write_lock
or try to generalize this code too much.

> > To me, the main question is: can we use synchronize_sched() in cpu_down?
> > It is slow.
> >
>
> Haha :-) So we don't want smp_mb() in the reader,

We need mb() + rmb(). Plust cli/sti unless this arch has optimized
this_cpu_add() like x86 (as you pointed out).

> *and* also don't want
> synchronize_sched() in the writer! Sounds like saying we want to have the cake
> and eat it too ;-) :P

Personally I'd vote for synchronize_sched() but I am not sure. And I do
not really understand the problem space.

> And moreover, since I'm still not convinced about the writer API part if use
> synchronize_sched(), I'd rather avoid synchronize_sched().)

Understand.

And yes, synchronize_sched() adds more problems. For example, where should
we call it? I do not this _cpu_down() should do this, in this case, say,
disable_nonboot_cpus() needs num_online_cpus() synchronize_sched's.

So probably cpu_down() should call it before cpu_maps_update_begin(), this
makes the locking even less obvious.

In short. What I am trying to say is, don't ask me I do not know ;)

Oleg.

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