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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_disable_ip(unsigned long ip, unsigned int cnt)
    > +{
    > + unsigned long flags;
    > + int newcnt;
    > +
    > + WARN_ON_ONCE(in_hardirq());
    > +
    > + /* First entry of a task into a BH disabled section? */
    > + if (!current->softirq_disable_cnt) {
    > + if (preemptible()) {
    > + local_lock(&softirq_ctrl.lock);
    > + rcu_read_lock();

    Ah you lock RCU because local_bh_disable() implies it and
    since it doesn't disable preemption anymore, you must do it
    explicitly?

    Perhaps local_lock() should itself imply rcu_read_lock() ?

    > + } else {
    > + DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));
    > + }
    > + }
    > +
    > + preempt_disable();

    Do you really need to disable preemption here? Migration is disabled by local_lock()
    and I can't figure out a scenario where the below can conflict with a preempting task.

    > + /*
    > + * Track the per CPU softirq disabled state. On RT this is per CPU
    > + * state to allow preemption of bottom half disabled sections.
    > + */
    > + newcnt = this_cpu_add_return(softirq_ctrl.cnt, cnt);

    __this_cpu_add_return() ?

    > + /*
    > + * Reflect the result in the task state to prevent recursion on the
    > + * local lock and to make softirq_count() & al work.
    > + */
    > + current->softirq_disable_cnt = newcnt;
    > +
    > + if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && newcnt == cnt) {
    > + raw_local_irq_save(flags);
    > + lockdep_softirqs_off(ip);
    > + raw_local_irq_restore(flags);
    > + }
    > + preempt_enable();
    > +}
    > +EXPORT_SYMBOL(__local_bh_disable_ip);
    > +
    > +static void __local_bh_enable(unsigned int cnt, bool unlock)
    > +{
    > + unsigned long flags;
    > + int newcnt;
    > +
    > + DEBUG_LOCKS_WARN_ON(current->softirq_disable_cnt !=
    > + this_cpu_read(softirq_ctrl.cnt));

    __this_cpu_read() ? Although that's lockdep only so not too important.

    > +
    > + preempt_disable();

    Same question about preempt_disable().

    > + if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && softirq_count() == cnt) {
    > + raw_local_irq_save(flags);
    > + lockdep_softirqs_on(_RET_IP_);
    > + raw_local_irq_restore(flags);
    > + }
    > +
    > + newcnt = this_cpu_sub_return(softirq_ctrl.cnt, cnt);

    __this_cpu_sub_return?

    > + current->softirq_disable_cnt = newcnt;
    > + preempt_enable();
    > +
    > + if (!newcnt && unlock) {
    > + rcu_read_unlock();
    > + local_unlock(&softirq_ctrl.lock);
    > + }
    > +}

    Thanks.

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