Re: Fix unlock_buffer() to work the same way as bit_unlock()

[Zoltan Menyhart <Zoltan.Menyhart@bull.net>]

Christoph Lameter wrote:
> Hmmm... Maybe we therefore need to add a mode to each bit operation in 
> the kernel?
>
> With that we can also get rid of the __* version of bitops.
>
> Possible modes are
>
> NON_ATOMIC 	Do not perform any atomic ops at all.
>
> ATOMIC		Atomic but unordered
>
> ACQUIRE		Atomic with acquire semantics (or lock semantics)
>
> RELEASE 	Atomic with release semantics (or unlock semantics)
>
> FENCE		Atomic with full fence.
>
> This would require another bitops overhaul.
>
> Maybe we can preserve the existing code with bitops like __* mapped to 
> *(..., NON_ATOMIC) and * mapped to *(..., FENCE) and the gradually fix the 
> rest of the kernel.

Form semantical point of view, the forms:

	bit_foo(..., mode)
and
	bit_foo_mode(...)

are equivalent.

However, I do not think your implementation would be efficient due to
selecting the ordering mode at run time:

> +	switch (mode) {
> +	case MODE_NONE :
> +	case MODE_ACQUIRE :
> +		return cmpxchg_acq(m, old, new);
> +	case MODE_FENCE :
> +		smp_mb();
> +		/* Fall through */
> +	case MODE_RELEASE :
> +		return cmpxchg_rel(m, old, new);

> +	if (mode == ORDER_NON_ATOMIC) {
> +		*m |= bit;
> +		return;
> +	}

etc.

In addition, we may want to inline these primitives...

A compile-time selection of the appropriate code sequence would help.

Thanks,

Zoltan
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