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    Subject[patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-05

    * Ingo Molnar <mingo@elte.hu> wrote:

    > hm, another thing: i think call_rcu() needs to take the read-lock.
    > Right now it assumes that it has the data structure private, but
    > that's only statistically true on PREEMPT_RT: another CPU may have
    > this CPU's RCU control structure in use. So IRQs-off (or preempt-off)
    > is not a guarantee to have the data structure, the read lock has to be
    > taken.

    i've reworked the code to use the read-lock to access the per-CPU data
    RCU structures, and it boots with 2 CPUs and PREEMPT_RT now. The -40-05
    patch can be downloaded from the usual place:

    http://redhat.com/~mingo/realtime-preempt/

    had to add two hacks though:

    static void rcu_advance_callbacks(struct rcu_data *rdp)
    {
    if (rdp->batch != rcu_ctrlblk.batch) {
    if (rdp->donetail) // HACK
    *rdp->donetail = rdp->waitlist;
    ...

    void fastcall call_rcu(struct rcu_head *head,
    void (*func)(struct rcu_head *rcu))
    [...]
    rcu_advance_callbacks(rdp);
    if (rdp->waittail) // HACK
    *rdp->waittail = head;
    ...

    without them it crashes during bootup.

    maybe we are better off with the completely unlocked read path and the
    long grace periods.

    Ingo
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