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Date:	Fri, 11 Dec 2009 08:54:08 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Dave Chinner <david@...morbit.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Zhang Rui <rui.zhang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Lachlan McIlroy <lachlan@....com>
Subject: Re: spinlock in completion_done() (was: Re: Async resume patch (was:
 Re: [GIT PULL] PM updates for 2.6.33))


* Dave Chinner <david@...morbit.com> wrote:

> On Thu, Dec 10, 2009 at 08:59:47AM +0100, Ingo Molnar wrote:
> > * Rafael J. Wysocki <rjw@...k.pl> wrote:
> > > On Wednesday 09 December 2009, Ingo Molnar wrote:
> > > > * Rafael J. Wysocki <rjw@...k.pl> wrote:
> > > > > On Tuesday 08 December 2009, Alan Stern wrote:
> > > > > > On Tue, 8 Dec 2009, Rafael J. Wysocki wrote:
> > > > > > 
> > > > > > > BTW, is there a good reason why completion_done() doesn't use spin_lock_irqsave
> > > > > > > and spin_unlock_irqrestore?  complete() and complete_all() use them, so why not
> > > > > > > here?
> > > > > > 
> > > > > > And likewise in try_wait_for_completion().  It looks like a bug.  Maybe 
> > > > > > these routines were not intended to be called with interrupts disabled, 
> > > > > > but that requirement doesn't seem to be documented.  And it isn't a 
> > > > > > natural requirement anyway.
> 
> When I implemented them they were not called from anywhere that
> disabled interrupts.  IIRC the main reason I used spin_lock_irq()
> was because that is what wait_for_completion() used at the time....

Obviously wait_for_competion() as a non-atomic API that can block will 
(and should) use _irq() - but atomic variants (complete, but also the 
try-wait thing) use irqsafe methods. A fair portion of completions 
happen in IRQ context.

	Ingo
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