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    SubjectRe: [PATCH] generic-ipi: make struct call_function_data lockless
    On Fri, 24 Jul 2009 17:50:16 +0800
    Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> wrote:

    > This patch can remove spinlock from struct call_function_data, the
    > reasons are below:
    >
    > 1: add a new interface for cpumask named cpumask_test_and_clear_cpu(),
    > it can atomically test and clear specific cpu, we can use it instead
    > of cpumask_test_cpu() and cpumask_clear_cpu() and no need data->lock
    > to protect those in generic_smp_call_function_interrupt().
    >
    > 2: in smp_call_function_many(), after csd_lock() return, the current's
    > cfd_data is deleted from call_function list, so it not have race
    > between other cpus, then cfs_data is only used in
    > smp_call_function_many() that must disable preemption and not from
    > a hardware interrupthandler or from a bottom half handler to call,
    > only the correspond cpu can use it, so it not have race in current
    > cpu, no need cfs_data->lock to protect it.
    >
    > 3: after 1 and 2, cfs_data->lock is only use to protect cfs_data->refs in
    > generic_smp_call_function_interrupt(), so we can define cfs_data->refs
    > to atomic_t, and no need cfs_data->lock any more.

    Looks good to me. One tiny cleanup:

    --- a/kernel/smp.c~generic-ipi-make-struct-call_function_data-lockless-cleanup
    +++ a/kernel/smp.c
    @@ -193,7 +193,7 @@ void generic_smp_call_function_interrupt

    data->csd.func(data->csd.info);

    - refs = atomic_sub_return(1, &data->refs);
    + refs = atomic_dec_return(&data->refs);
    WARN_ON(refs < 0);
    if (!refs) {
    spin_lock(&call_function.lock);
    _



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