workqueue, irq_work: Build fix for !CONFIG_IRQ_WORK

2f34d733 ("workqueue: Fix queue_work_on() with BH workqueues") added
irq_work usage to workqueue; however, it turns out irq_work is actually
optional and the change breaks build on configuration which doesn't have
CONFIG_IRQ_WORK enabled.

Fix build by making workqueue use irq_work only when CONFIG_SMP and enabling
CONFIG_IRQ_WORK when CONFIG_SMP is set. It's reasonable to argue that it may
be better to just always enable it. However, this still saves a small bit of
memory for tiny UP configs and also the least amount of change, so, for now,
let's keep it conditional.

Verified to do the right thing for x86_64 allnoconfig and defconfig, and
aarch64 allnoconfig, allnoconfig + prink disable (SMP but nothing selects
IRQ_WORK) and a modified aarch64 Kconfig where !SMP and nothing selects
IRQ_WORK.

v2: `depends on SMP` leads to Kconfig warnings when CONFIG_IRQ_WORK is
    selected by something else when !CONFIG_SMP. Use `def_bool y if SMP`
    instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Fixes: 2f34d733 ("workqueue: Fix queue_work_on() with BH workqueues")
Cc: Stephen Rothwell <sfr@canb.auug.org.au>

init/Kconfig

@@ -106,7 +106,7 @@ config CONSTRUCTORS
 	bool
 
 config IRQ_WORK
-	bool
+	def_bool y if SMP
 
 config BUILDTIME_TABLE_SORT
 	bool

kernel/workqueue.c

@@ -1209,6 +1209,20 @@ static struct irq_work *bh_pool_irq_work(struct worker_pool *pool)
 	return &per_cpu(bh_pool_irq_works, pool->cpu)[high];
 }
 
+static void kick_bh_pool(struct worker_pool *pool)
+{
+#ifdef CONFIG_SMP
+	if (unlikely(pool->cpu != smp_processor_id())) {
+		irq_work_queue_on(bh_pool_irq_work(pool), pool->cpu);
+		return;
+	}
+#endif
+	if (pool->attrs->nice == HIGHPRI_NICE_LEVEL)
+		raise_softirq_irqoff(HI_SOFTIRQ);
+	else
+		raise_softirq_irqoff(TASKLET_SOFTIRQ);
+}
+
 /**
  * kick_pool - wake up an idle worker if necessary
  * @pool: pool to kick
@@ -1227,15 +1241,7 @@ static bool kick_pool(struct worker_pool *pool)
 		return false;
 
 	if (pool->flags & POOL_BH) {
-		if (likely(pool->cpu == smp_processor_id())) {
-			if (pool->attrs->nice == HIGHPRI_NICE_LEVEL)
-				raise_softirq_irqoff(HI_SOFTIRQ);
-			else
-				raise_softirq_irqoff(TASKLET_SOFTIRQ);
-		} else {
-			irq_work_queue_on(bh_pool_irq_work(pool), pool->cpu);
-		}
-
+		kick_bh_pool(pool);
 		return true;
 	}