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    SubjectRe: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

    Great work on finding this!

    On Mon, 2006-10-09 at 18:09 -0700, Tim Chen wrote:

    > The original "C" code looks very innocent:
    > if (WARN_ON(__ret_warn_once));
    > __warn_once = 0;
    > The equivalent asm code generated by gcc looks like:
    > temp = 0;
    > if (!WARN_ON(__ret_warn_once))
    > temp = __warn_once;
    > __warn_once = temp;
    > As a result, a global variable is being written from all CPUs
    > everywhere and caused excessive cache line bouncing on SMP.
    > We measured that HITM event increased by 75% and
    > read-for-ownership event increased by 50%. Adding a
    > __read_mostly directive to __warn_once didn't help
    > because gcc still generate assembly code that write to
    > that global variable.

    Holy crap! I wonder where else in the kernel gcc is doing this. (of
    course I'm using gcc4 so I don't know). Is there another gcc attribute
    to actually tell gcc that a variable is really mostly read only (besides
    placing it in a mostly read only elf section)?

    What was wrong with the original WARN_ON_ONCE with

    if (unlikely(condition) && __warn_once)

    This didn't have the cache crash problem too, did it?
    I don't have a gcc3 around to test.

    -- Steve

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