Re: [patch 01/23] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486

Mathieu Desnoyers
2007-08-20T20:41:41Z

* Christoph Lameter (clameter@sgi.com) wrote:
> On Mon, 20 Aug 2007, Mathieu Desnoyers wrote:
>
> > already present in cmpxchg_386_u32 tells much about how this cmpxchg
> > implementation should not be used in a SMP context. However, the cmpxchg_local
> > can perfectly use this fallback, since it only needs to be atomic wrt the local
> > cpu.
>
> Do you have an example where the use of cmpxchg_local is a performance
> benefit when actually used in code? cmpxchg is already problematic
> functionality since it is not available on all platforms.

Yes, I use it as synchronization mechanism for my buffer management algorithm
in LTTng. Since I write in per-cpu buffers and want to be as reentrant
as possible wrt other contexts (dealing with NMI as worse case, but also
applies to MCE..), I use local_cmpxchg to reserve space in my buffers.
It is faster than the standard cmpxchg.

Mathieu