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

[Peter Hartley]

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


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