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    SubjectRe: [RFC][PATCH 16/17] sched: Move the second half of ttwu() to the remote cpu
    On 12/24, Peter Zijlstra wrote:
    >
    > +static void
    > +ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags)
    > +{
    > +#ifdef CONFIG_SMP
    > + if (task_cpu(p) != cpu_of(rq))
    > + set_task_cpu(p, cpu_of(rq));
    > +#endif

    This looks a bit suspicious.

    If this is called by sched_ttwu_pending() we are holding rq->lock,
    not task_rq_lock(). It seems, we can race with, say, migration
    thread running on task_cpu().



    OK, p->state = TASK_WAKING protects us against, say, set_cpus_allowed_ptr()
    which does task_rq_lock(p) and thus checks task_is_waking().

    But, at the same time,

    > +#ifdef CONFIG_SMP
    > +static void ttwu_queue_remote(struct task_struct *p, int cpu)
    > +{
    > + struct task_struct *next = NULL;
    > + struct rq *rq = cpu_rq(cpu);
    > +
    > + for (;;) {
    > + struct task_struct *old = next;
    > +
    > + p->wake_entry = next;
    > + next = cmpxchg(&rq->wake_list, old, p);
    > + if (next == old)
    > + break;
    > + }
    > +
    > + if (!next)
    > + smp_send_reschedule(cpu);

    what if that cpu does set_cpus_allowed_ptr(p) ?

    It spins with irq disabled. Once the caller, try_to_wake_up(),
    drops ->pi_lock it will wait for !task_is_waking() forever, no?

    Oleg.



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