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

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Message-Id: <200803050054.04930.rjw@sisk.pl>
Date:	Wed, 5 Mar 2008 00:54:03 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Peter Hartley <pdh@...er.chaos.org.uk>
Cc:	linux-kernel@...r.kernel.org, Pavel Machek <pavel@....cz>,
	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,
	Pierre Ossman <drzeus-mmc@...eus.cx>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read is required

On Wednesday, 5 of March 2008, Peter Hartley wrote:
> 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..."

Well, perhaps it's sufficient to document just pointers?  In fact this is what
RCU relies on.

Thanks,
Rafael
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