Re: [patch 09/10] SLUB: Do our own locking via slab_lock and slab_unlock.

Christoph Lameter
2007-10-30T18:32:51Z

On Tue, 30 Oct 2007, Nick Piggin wrote:

> Is this actually a speedup on any architecture to roll your own locking
> rather than using bit spinlock?

It avoids one load from memory when allocating and the release is simply
writing the page->flags back. Less instructions.

> I am not exactly convinced that smp_wmb() is a good idea to have in your
> unlock, rather than the normally required smp_mb() that every other open
> coded lock in the kernel is using today. If you comment every code path
> where a load leaking out of the critical section would not be a problem,
> then OK it may be correct, but I still don't think it is worth the
> maintenance overhead.

I thought you agreed that release semantics only require a write barrier?
The issue here is that other processors see the updates before the
updates to page-flags.

A load leaking out of a critical section would require that the result of
the load is not used to update other information before the slab_unlock
and that the source of the load is not overwritten in the critical
section. That does not happen in sluib.