linux-kernel - Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

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Message-Id: <200912082252.51916.borntraeger@de.ibm.com>
Date:	Tue, 8 Dec 2009 22:52:51 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, 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>
Subject: Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

> > Sure they can. Control dependencies are trivial - it's called "branch 
> > prediction", and everybody does it, and data dependencies don't exist on 
> > many CPU architectures (even to the point of reading through a pointer 
> > that you loaded).
> 
> Wait a second.  Are you saying that with code like this:
> 
> 	if (x == 1)
> 		y = 5;
> 
> the CPU may write to y before it has finished reading the value of x?  
> And this write is visible to other CPUs, so that if x was initially 0
> and a second CPU sets x to 1, the second CPU may see y == 5 before it
> executes the write to x (whatever that may mean)?

No, the write really depends on x being 1 at any time before the comparison.
On the other hand x being != 0 during the comparison does not prevent the 
write without proper locking or barriers.


Have a look at

http://www.linuxjournal.com/article/8211
http://www.linuxjournal.com/article/8212

especially at the alpha part what can happen when dealing with pointer accesses.

Christian
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