Re: [PATCH net-next v1] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT

On Thu, 18 Dec 2025 13:25:31 +0100
Paolo Abeni [off-list ref] wrote:

AFAICS, this part is not needed: local_bh_disable() ensures migrating is
already disabled, if !CONFIG_PREEMPT_RT_NEEDS_BH_LOCK or preemption is
disabled, when CONFIG_PREEMPT_RT_NEEDS_BH_LOCK==y

As the code has this:

	/* First entry of a task into a BH disabled section? */
	if (!current->softirq_disable_cnt) {
		if (preemptible()) {
			if (IS_ENABLED(CONFIG_PREEMPT_RT_NEEDS_BH_LOCK))
				local_lock(&softirq_ctrl.lock);
			else
				migrate_disable();

			/* Required to meet the RCU bottomhalf requirements. */
			rcu_read_lock();
		} else {
			DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));
		}
	}

It looks as though migration will always be disabled (local_lock() also
disables migration). It will warn if preemption is disabled.

But yeah, the added migrate_disable() is not needed.

-- Steve