linux-kernel - Re: atomics: document that linux expects certain atomic behaviour from unsigned long

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Message-ID: <20090105112350.3e665114@lxorguk.ukuu.org.uk>
Date:	Mon, 5 Jan 2009 11:23:50 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Pavel Machek <pavel@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org
Subject: Re: atomics: document that linux expects certain atomic behaviour
 from unsigned long

> Pretty much everywhere that uses RCU for example does so using atomic pointer
> loads and stores. The nastiest issue IMO actually is reloading the value
> through the pointer even if it isn't explicitly dereferenced. RCU gets this
> right with ACCESS_ONCE. Probably a lot of code using basic types does not.
> x86 atomic_read maybe should be using ACCESS_ONCE too...

I'm pretty sure it should. gcc makes no guarantees about not being clever
with accesses.

Alan
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