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    SubjectRe: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers
    On 10/23, Oleg Nesterov wrote:
    >
    > Not really the comment, but the question...

    Damn. And another question.

    Mikulas, I am sorry for this (almost) off-topic noise. Let me repeat
    just in case that I am not arguing with your patches.




    So write_lock/write_unlock needs to call synchronize_sched() 3 times.
    I am wondering if it makes any sense to try to make it a bit heavier
    but faster.

    What if we change the reader to use local_irq_disable/enable around
    this_cpu_inc/dec (instead of rcu read lock)? I have to admit, I have
    no idea how much cli/sti is slower compared to preempt_disable/enable.

    Then the writer can use

    static void mb_ipi(void *arg)
    {
    smp_mb(); /* unneeded ? */
    }

    static void force_mb_on_each_cpu(void)
    {
    smp_mb();
    smp_call_function(mb_ipi, NULL, 1);
    }

    to a) synchronise with irq_disable and b) to insert the necessary mb's.

    Of course smp_call_function() means more work for each CPU, but
    write_lock() should be rare...

    This can also wakeup the idle CPU's, but probably we can do
    on_each_cpu_cond(cond_func => !idle_cpu). Perhaps cond_func() can
    also return false if rcu_user_enter() was called...

    Actually I was thinking about this from the very beginning, but I do
    not feel this looks like a good idea. Still I'd like to ask what do
    you think.

    Oleg.



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