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

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Message-ID: <Pine.LNX.4.44L0.0912081626400.3046-100000@iolanthe.rowland.org>
Date:	Tue, 8 Dec 2009 16:32:47 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	"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)

On Tue, 8 Dec 2009, Linus Torvalds wrote:

> > No, that's not needed.  Unlike reads, writes can't move in front of
> > data or control dependencies.  Or so I've been lead to believe...
> 
> 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)?

Alan Stern

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