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

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Date:	Sat, 3 Jan 2009 23:14:19 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Pavel Machek <pavel@...e.cz>
Cc:	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

> Is there concrete architecture where it breaks? I'd expect i386/x86-64
> to be safe, and pretty much everyone to be safe as long as that long
> is aligned.... or that was the result of arch-maintainers
> discussion...

It'll break on x86 if gcc decides to cache the value and you don't have
explicit barriers. If the long is not aligned it's not safe on x86 at all.

> I'd really like to document if it is right or not, so that I can point
> people to documentation...

We should always tell people to use atomic/set_bit etc. There *are* cases
you can get away with it but it is far far better that the default is the
safe one because most driver writers do not have a detailed knowledge of
gcc code generation, processor quirks and barriers. If in a specific case
its a performance hit then its worth optimising that case.

Alan
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