RE: Synchronizing Bit operations V2

["Chen, Kenneth W" <kenneth.w.chen@intel.com>]

Christoph Lameter wrote on Thursday, March 30, 2006 4:18 PM
> Note that the current semantics for bitops IA64 are broken. Both
> smp_mb__after/before_clear_bit are now set to full memory barriers
> to compensate

Why you say that?  clear_bit has built-in acq or rel semantic depends
on how you define it. I think only one of smp_mb__after/before need to
be smp_mb?



>  static __inline__ void
>  clear_bit (int nr, volatile void *addr)
>  {
> -	__u32 mask, old, new;
> -	volatile __u32 *m;
> -	CMPXCHG_BUGCHECK_DECL
> -
> -	m = (volatile __u32 *) addr + (nr >> 5);
> -	mask = ~(1 << (nr & 31));
> -	do {
> -		CMPXCHG_BUGCHECK(m);
> -		old = *m;
> -		new = old & mask;
> -	} while (cmpxchg_acq(m, old, new) != old);
> +	clear_bit_mode(nr, addr, MODE_ATOMIC);
>  }

I would make that MODE_RELEASE for clear_bit, simply to match the
observation that clear_bit is usually used in unlock path and have
potential less surprises.


> +static __inline__ void
> +set_bit_mode (int nr, volatile void *addr, int mode)
> +{
> +	__u32 bit, old, new;
> +	volatile __u32 *m;
> +	CMPXCHG_BUGCHECK_DECL
> +
> +	m = (volatile __u32 *) addr + (nr >> 5);
> +	bit = 1 << (nr & 31);
> +
> +	if (mode = MODE_NON_ATOMIC) {
> +		*m |= bit;
> +		return;
> +	}

Please kill all volatile declaration, because for non-atomic version,
you don't need to do any memory ordering, but compiler automatically
adds memory order because of volatile.  It's safe to kill them because
cmpxchg later has explicit mode in there.

Same thing goes to all other bit ops.

- Ken