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    SubjectRe: [PATCH 2/5] atomic: introduce atomic_inc_not_zero
    Roman Zippel wrote:
    > Hi,
    > On Thu, 15 Sep 2005, Nick Piggin wrote:
    >>Roman: any ideas about what you would prefer? You'll notice
    >>atomic_inc_not_zero replaces rcuref_inc_lf, which is used several times
    >>in the VFS.
    > In the larger picture I'm not completely happy with these scalibilty
    > patches, as they add extra overhead at the lower end. On a UP system in
    > general nothing beats:
    > spin_lock();
    > if (*ptr)
    > ptr += 1;
    > spin_unlock();
    > The main problem is here that the atomic functions are used in two basic
    > situation:
    > 1) interrupt synchronization
    > 2) multiprocessor synchronization
    > The atomic functions have to assume both, but on UP systems it often is
    > a lot cheaper if they don't have to synchronize with interrupts. So
    > replacing a spinlock with a few atomic operations can hurt UP performance.

    Maybe so, but what I'm doing is introducing a slightly better
    implementation of what is currently in tree, and attempting to
    follow current standards as far as possible. I don't think you
    could say that is a bad thing.

    Now I don't think anyone would be flat out opposed to 1 - reworking
    the atomic.h code to allow some genericity (is that a word?); 2 -
    reworking atomic.h code to allow combining of atomic ops, or allowing
    interrupt unsafe ops...

    Of course, neither is going to be merged unless done tastefully, and
    I imagine both would be difficult to get right, with probably a low
    cost/benefit ratio.


    SUSE Labs, Novell Inc.

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