Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

Jarek Poplawski
2009-07-08T07:18:54Z

On Wed, Jul 08, 2009 at 12:34:32AM -0400, Mathieu Desnoyers wrote:
...
> Because adding smp_mb__after_lock() is _only_ useful on x86. Most other
> architectures _will_ suffer from a performance degradation, unless you
> implement the __read_lock_noacquire.

It's completely backwards: processor barriers are just expected to
add a performance degradation. That's like:

x86 developer:
OK, we need to add a barrier here: even x86 might need this.

Alpha developer:
Right, than we need this even more.

x86 developer:
But wait, we can avoid it using a dummy after some locks, because they
have such a barrier already.

Alpha developer:
Then it's not OK: it's _only_ useful on x86; our architecture _will_
suffer from a performance degradation...

Cheers,
Jarek P.