Re: [patch] Re: using long instead of atomic_t when only set/read is required

Paul E. McKenney
2008-03-05T00:26:39Z

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