Re: [patch 2/2] powerpc: native atomic_add_unless

[Joel Schopp <jschopp@austin.ibm.com>]

> I didn't convert LL/SC architectures so as to "encourage" them
> to do atomic_add_unless natively. Here is my probably-wrong attempt
> for powerpc.
> 
> Should I bring this up on the arch list? (IIRC cross posting 
> between there and lkml is discouraged)
> 

You should bring this up on the arch list or it is likely to be missed by people 
who would give you useful feedback.

> +static __inline__ int atomic_add_unless(atomic_t *v, int a, int u)
> +{
> +	int t;
> +	int dummy;
> +
> +	__asm__ __volatile__ (
> +	LWSYNC_ON_SMP

I realize to preserve the behavior you currently get with the cmpxchg currently 
used to implement atomic_add_unless that you feel the need to put in an lwsync & 
isync.  But I would point out that neither is necessary to actually atomic add 
unless.  They are simply so cmpxchg can be overloaded to be used as both a lock 
and unlock primitive.  If atomic_add_unless isn't being used as a locking 
primitive somewhere I would love for these to be dropped.

> +"1:	lwarx	%0,0,%2		# atomic_add_unless\n\
> +	cmpw	0,%0,%4 \n\

This is just personal preference, but I find it more readable to use the 
simplified mnemonic:
cmpw %0, %4

> +	beq-	2f \n\
> +	add	%1,%3,%0 \n"

Why use a separate register here? Why not reuse %0 instead of using %1? 
Registers are valuable.

> +	PPC405_ERR77(0,%2)
> +"	stwcx.	%1,0,%2 \n\
> +	bne-	1b \n"
> +	ISYNC_ON_SMP
> +	"\n\
> +2:"
> +	: "=&r" (t), "=&r" (dummy)
> +	: "r" (&v->counter), "r" (a), "r" (u)
> +	: "cc", "memory");
> +
> +	return likely(t != u);
> +}
> +
>  #define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)