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

Rafael J. Wysocki
2008-03-04T23:56:17Z

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