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    SubjectRe: [patch 14/19] softirq: Make softirq control and processing RT aware
    On Fri, Nov 13, 2020 at 03:02:21PM +0100, Thomas Gleixner wrote:
    > +void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
    > +{
    > + bool preempt_on = preemptible();
    > + unsigned long flags;
    > + u32 pending;
    > + int curcnt;
    > +
    > + WARN_ON_ONCE(in_irq());
    > + lockdep_assert_irqs_enabled();
    > +
    > + local_irq_save(flags);
    > + curcnt = this_cpu_read(softirq_ctrl.cnt);
    > +
    > + /*
    > + * If this is not reenabling soft interrupts, no point in trying to
    > + * run pending ones.
    > + */
    > + if (curcnt != cnt)
    > + goto out;
    > +
    > + pending = local_softirq_pending();
    > + if (!pending || ksoftirqd_running(pending))
    > + goto out;
    > +
    > + /*
    > + * If this was called from non preemptible context, wake up the
    > + * softirq daemon.
    > + */
    > + if (!preempt_on) {
    > + wakeup_softirqd();
    > + goto out;
    > + }
    > +
    > + /*
    > + * Adjust softirq count to SOFTIRQ_OFFSET which makes
    > + * in_serving_softirq() become true.
    > + */
    > + cnt = SOFTIRQ_OFFSET;
    > + __local_bh_enable(cnt, false);

    But then you enter __do_softirq() with softirq_count() == SOFTIRQ_OFFSET.
    __do_softirq() calls softirq_handle_begin() which then sets it back to SOFTIRQ_DISABLE_OFFSET...

    > + __do_softirq();
    > +
    > +out:
    > + __local_bh_enable(cnt, preempt_on);

    You escape from there with a correct preempt_count() but still the softirq executes
    under SOFTIRQ_DISABLE_OFFSET and not SOFTIRQ_OFFSET, making in_serving_softirq() false.

    > + local_irq_restore(flags);
    > +}
    > +EXPORT_SYMBOL(__local_bh_enable_ip);

    Thanks.

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