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    SubjectRe: [PATCH for 2.5] preemptible kernel
    "David S. Miller" wrote:
    >
    > Keith Owens writes:
    > > Or have I missed something?
    >
    > Nope, it is a fundamental problem with such kernel pre-emption
    > schemes. As a result, it would also break our big-reader locks
    > (see include/linux/brlock.h).
    >
    > Basically, anything which uses smp_processor_id() would need to
    > be holding some lock so as to not get pre-empted.
    >

    It's a problem for uniprocessors as well.

    Example:

    #define current_cpu_data boot_cpu_data
    #define pgd_quicklist (current_cpu_data.pgd_quick)

    extern __inline__ void free_pgd_fast(pgd_t *pgd)
    {
    *(unsigned long *)pgd = (unsigned long) pgd_quicklist;
    pgd_quicklist = (unsigned long *) pgd;
    pgtable_cache_size++;
    }

    Preemption could corrupt this list.

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