Re: page table lock patch V15 [0/7]: overview

Andi Kleen
2005-01-14T04:14:21Z

On Fri, Jan 14, 2005 at 04:39:16AM +0100, Roman Zippel wrote:
> Hi,
>
> Christoph Lameter wrote:
>
> >Introduction of the cmpxchg is one atomic operations that replaces the two
> >spinlock ops typically necessary in an unpatched kernel. Obtaining the
> >spinlock requires an spinlock (which is an atomic operation) and then the
> >release involves a barrier. So there is a net win for all SMP cases as far
> >as I can see.
>
> But there might be a loss in the UP case. Spinlocks are optimized away,
> but your cmpxchg emulation enables/disables interrupts with every access.

Only for 386s and STI/CLI is quite cheap there.

-Andi