linux-kernel - using long instead of atomic_t when only set/read is required (was Re: [Bug 10030] Suspend doesn't work when SD card is inserted)

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Message-ID: <20080225090316.GA420@elf.ucw.cz>
Date:	Mon, 25 Feb 2008 10:03:16 +0100
From:	Pavel Machek <pavel@....cz>
To:	Alan Stern <stern@...land.harvard.edu>, zdenek.kabelac@...il.com,
	davem@...emloft.net
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Pierre Ossman <drzeus-mmc@...eus.cx>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: using long instead of atomic_t when only set/read is required (was
	Re: [Bug 10030] Suspend doesn't work when SD card is inserted)

Hi!

Alan thinks that `subj` is correct...

> > > > At the very least, you'd need rmb() before reading it and wmb() after
> > > > writing to it, but I'm not sure if that's enough on every obscure
> > > > architecture out there.
> > > 
> > > No, neither one is needed because of the way suspending_task is used.  
> > > 
> > > It's not necessary for a reader R to see the variable's actual value;  
> > > all R needs to know is whether or not suspending_task is equal to R.  
> > > Since the only process which can set suspending_task to R is R itself,
> > > and since R will set suspending_task back to NULL before releasing the
> > > write lock on pm_sleep_rwsem, there's never any ambiguity.
> > 
> > Subtle.
> > 
> > Very subtly wrong ;-).
> > 
> > imagine suspending_task == 0xabcdef01. Now task "R" with current ==
> > 0xabcd0000 reads suspending_task while the other cpu is writing to it,
> > and sees 0xabcd0000 (0xef01 was not yet written) -- and mistakenly
> > believes that  "R" == suspending_task.
> 
> I always thought that reads and writes of pointers are atomic, just 
> like reads and writes of longs.  Is that wrong?

...but I'm not that sure. Can someone clarify?

I guess it only works as long as longs are aligned? Should it be
written down to atomic_ops.txt?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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