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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20090103231419.197727ba@lxorguk.ukuu.org.uk>
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/