linux-kernel - Re: [linux-pm] [PATCH 1/9] PM: Add suspend block api.

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Message-ID: <Pine.LNX.4.44L0.1004231244090.1777-100000@iolanthe.rowland.org>
Date:	Fri, 23 Apr 2010 12:45:41 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Arve Hjønnevåg <arve@...roid.com>
cc:	Len Brown <len.brown@...el.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Magnus Damm <damm@...l.co.jp>,
	<linux-pm@...ts.linux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH 1/9] PM: Add suspend block api.

On Fri, 23 Apr 2010, Alan Stern wrote:

> On Thu, 22 Apr 2010, [UTF-8] Arve HjÃ¸nnevÃ¥g wrote:

> > +struct suspend_blocker {
> > +#ifdef CONFIG_SUSPEND_BLOCKERS
> > +	atomic_t            flags;
> > +	const char         *name;
> > +#endif
> 
> Why is flags an atomic_t?  Are you worried that drivers might try to 
> activate a suspend_blocker at the same time that it is being destroyed?
> If this happens, does the code do the right thing?  I don't think it 
> does -- if a race occurs, suspend_block() will leave flags set to the 
> wrong value.  The same goes for suspend_unblock().
> 
> Since these routines don't nest, there is also the possibility of a
> race between suspend_block() and suspend_unblock().  If the race goes
> one way the blocker is active; the other way it isn't.  Given that such
> problems already exist, why worry about what happens when the suspend
> blocker is destroyed?

Having now read the later patches, I see that you switch over to using 
a spinlock instead of an atomic_t.  My suggestion is to use a spinlock 
right from the start.  It will be less confusing.

Alan Stern

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