Date: Tue, 4 Mar 2008 16:26:26 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Peter Hartley <pdh@utter.chaos.org.uk>, linux-kernel@vger.kernel.org,
       Pavel Machek <pavel@ucw.cz>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Alan Stern <stern@rowland.harvard.edu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@osdl.org>,
       Zdenek Kabelac <zdenek.kabelac@gmail.com>, davem@davemloft.net,
       Pierre Ossman <drzeus-mmc@drzeus.cx>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read is required
Message-ID: <20080305002626.GD9517@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Mar 05, 2008 at 12:54:03AM +0100, Rafael J. Wysocki wrote:
> 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.

One can do RCU on array indexes (ints/longs) as well as pointers, so we
need to keep "integral" in there.

							Thanx, Paul
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