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

    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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