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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