linux-kernel - Re: [patch] Re: using long instead of atomic

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: <1204673572.1020.12.camel@amd64.pyotr.org>
Date:	Tue, 04 Mar 2008 23:32:52 +0000
From:	Peter Hartley <pdh@...er.chaos.org.uk>
To:	linux-kernel@...r.kernel.org, Pavel Machek <pavel@....cz>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Alan Stern <stern@...land.harvard.edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...l.org>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>, davem@...emloft.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Pierre Ossman <drzeus-mmc@...eus.cx>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read
	is required

On Mon, 2008-03-03 at 18:24 +0100, Pavel Machek wrote:
> Ok, I can understand the gcc side. But do we actually run on an
> architecture where
> 
> long *x;
> 
> *x = 0;
> 
> racing with 
> 
> *x = 0x12345678;
> 
> can produce
> 
> *x == 0x12340000;
> 
> or something like that? I'm told RCU relies on architectures not doing
> this, and I'd like to get this clarified.

ARM6, ARM7500 and similar do exactly this for short (and unsigned
short), although not for int, long, or pointers:

> struct foo { short b; short c; };
> void baa(struct foo *f, short cc) { f->c = cc; }

becomes (arm-linux-gcc -mcpu=arm6):

> baa:
>	mov	r3, r1, lsr #8
>	strb	r3, [r0, #3]
>	strb	r1, [r0, #2]
>	mov	pc, lr

note the two single-byte stores, as ARM6 didn't have the "store
halfword" instruction.

So I think Alan Stern's
"For all properly-aligned pointer and integral types other than long
long..."
should be amended to
"For all properly-aligned pointer and integral types other than short or
long long..."

	Peter


--
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/