linux-kernel - Re: [PATCH 1/8] PM: Add suspend block api.

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Date:	Fri, 7 May 2010 10:22:20 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Arve Hjønnevåg <arve@...roid.com>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...otime.net>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Ming Lei <tom.leiming@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	<linux-doc@...r.kernel.org>
Subject: Re: [PATCH 1/8] PM: Add suspend block api.

On Thu, 6 May 2010, Arve Hjønnevåg wrote:

> On Thu, May 6, 2010 at 12:40 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Thu, 6 May 2010, Rafael J. Wysocki wrote:
> >
> >> > Here's a completely new issue.  When using opportunistic suspends on an
> >> > SMP system, it could happen that the system gets a wakeup event and
> >> > this routine starts running again before the event's IRQ handler has
> >> > finished (or has enabled a suspend blocker).  The system would
> >> > re-suspend too soon.
> >>
> >> This routine will be run from a freezable workqueue.
> >
> > But how do you know that processes won't get unfrozen until all the
> > pending IRQs have been handled?  Imagine something like this:
> >
> >        CPU 0                   CPU 1
> >        -----                   -----
> >        Wake up non-boot CPUs
> >        Resume devices          Invoke the IRQ handler
> >
> >        [ CPU 0 should wait here for the handler to finish,
> >          but it doesn't ]
> >
> >        Defrost threads         Handler running...
> >        Workqueue routine runs
> >        Start another suspend
> >                                Handler enables a suspend blocker,
> >                                but it's too late
> 
> It is not optimal, but it is not too late. We check if any suspend
> blockers block suspend after disabling non-boot cpus so as long as
> this is done in a way that does not lose interrupts the resuspend
> attempt will not succeed.

Is it possible for the resuspend to disable CPU 1 before the IRQ
handler can enable its suspend blocker?  (Probably not -- but I don't
know enough about how non-boot CPUs are enabled or disabled.)

Alan Stern

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