Subject: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue
Date:	Thu, 16 Apr 2015 18:28:16 +0200
From:	Jan Kiszka <jan.kiszka@siemens.com>

Instead of turning all irq_work requests into lazy ones on -rt, just
move their execution from hard into soft-irq context.

This resolves deadlocks of ftrace which will queue work from arbitrary
contexts, including those that have locks held that are needed for
raising a soft-irq.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

Second try, looks much better so far. And it also removes my concerns
regarding other potential cases besides ftrace.

 kernel/irq_work.c |   84 ++++++++++++++++++++++++++----------------------------
 1 file changed, 41 insertions(+), 43 deletions(-)

--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -80,17 +80,12 @@ bool irq_work_queue_on(struct irq_work *
 	if (!irq_work_claim(work))
 		return false;
 
-#ifdef CONFIG_PREEMPT_RT_FULL
-	if (work->flags & IRQ_WORK_HARD_IRQ)
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && (work->flags & IRQ_WORK_HARD_IRQ))
 		raise_irqwork = llist_add(&work->llnode,
 					  &per_cpu(hirq_work_list, cpu));
 	else
 		raise_irqwork = llist_add(&work->llnode,
-					  &per_cpu(lazy_list, cpu));
-#else
-		raise_irqwork = llist_add(&work->llnode,
 					  &per_cpu(raised_list, cpu));
-#endif
 
 	if (raise_irqwork)
 		arch_send_call_function_single_ipi(cpu);
@@ -103,6 +98,9 @@ EXPORT_SYMBOL_GPL(irq_work_queue_on);
 /* Enqueue the irq work @work on the current CPU */
 bool irq_work_queue(struct irq_work *work)
 {
+	bool realtime = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
+	bool raise = false;
+
 	/* Only queue if not already pending */
 	if (!irq_work_claim(work))
 		return false;
@@ -110,25 +108,22 @@ bool irq_work_queue(struct irq_work *wor
 	/* Queue the entry and raise the IPI if needed. */
 	preempt_disable();
 
-#ifdef CONFIG_PREEMPT_RT_FULL
-	if (work->flags & IRQ_WORK_HARD_IRQ) {
+	if (realtime && (work->flags & IRQ_WORK_HARD_IRQ)) {
 		if (llist_add(&work->llnode, this_cpu_ptr(&hirq_work_list)))
-			arch_irq_work_raise();
-	} else {
-		if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
-		    tick_nohz_tick_stopped())
-			raise_softirq(TIMER_SOFTIRQ);
-	}
-#else
-	if (work->flags & IRQ_WORK_LAZY) {
+			raise = 1;
+	} else if (work->flags & IRQ_WORK_LAZY) {
 		if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
-		    tick_nohz_tick_stopped())
-			arch_irq_work_raise();
-	} else {
-		if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
-			arch_irq_work_raise();
-	}
-#endif
+		    tick_nohz_tick_stopped()) {
+			if (realtime)
+				raise_softirq(TIMER_SOFTIRQ);
+			else
+				raise = true;
+		}
+	} else if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
+		raise = true;
+
+	if (raise)
+		arch_irq_work_raise();
 
 	preempt_enable();
 
@@ -143,12 +138,13 @@ bool irq_work_needs_cpu(void)
 	raised = this_cpu_ptr(&raised_list);
 	lazy = this_cpu_ptr(&lazy_list);
 
-	if (llist_empty(raised))
-		if (llist_empty(lazy))
-#ifdef CONFIG_PREEMPT_RT_FULL
+	if (llist_empty(raised) && llist_empty(lazy)) {
+		if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
 			if (llist_empty(this_cpu_ptr(&hirq_work_list)))
-#endif
 				return false;
+		} else
+			return false;
+	}
 
 	/* All work should have been flushed before going offline */
 	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
@@ -162,9 +158,7 @@ static void irq_work_run_list(struct lli
 	struct irq_work *work;
 	struct llist_node *llnode;
 
-#ifndef CONFIG_PREEMPT_RT_FULL
-	BUG_ON(!irqs_disabled());
-#endif
+	BUG_ON(!IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && !irqs_disabled());
 
 	if (llist_empty(list))
 		return;
@@ -200,26 +194,30 @@ static void irq_work_run_list(struct lli
  */
 void irq_work_run(void)
 {
-#ifdef CONFIG_PREEMPT_RT_FULL
-	irq_work_run_list(this_cpu_ptr(&hirq_work_list));
-#else
-	irq_work_run_list(this_cpu_ptr(&raised_list));
-	irq_work_run_list(this_cpu_ptr(&lazy_list));
-#endif
+	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
+		irq_work_run_list(this_cpu_ptr(&hirq_work_list));
+		/*
+		 * NOTE: we raise softirq via IPI for safety,
+		 * and execute in irq_work_tick() to move the
+		 * overhead from hard to soft irq context.
+		 */
+		if (!llist_empty(this_cpu_ptr(&raised_list)))
+			raise_softirq(TIMER_SOFTIRQ);
+	} else {
+		irq_work_run_list(this_cpu_ptr(&raised_list));
+		irq_work_run_list(this_cpu_ptr(&lazy_list));
+	}
 }
 EXPORT_SYMBOL_GPL(irq_work_run);
 
 void irq_work_tick(void)
 {
-#ifdef CONFIG_PREEMPT_RT_FULL
-	irq_work_run_list(this_cpu_ptr(&lazy_list));
-#else
-	struct llist_head *raised = &__get_cpu_var(raised_list);
+	struct llist_head *raised = this_cpu_ptr(&raised_list);
 
-	if (!llist_empty(raised) && !arch_irq_work_has_interrupt())
+	if (!llist_empty(raised) && (!arch_irq_work_has_interrupt() ||
+	    IS_ENABLED(CONFIG_PREEMPT_RT_FULL)))
 		irq_work_run_list(raised);
-	irq_work_run_list(&__get_cpu_var(lazy_list));
-#endif
+	irq_work_run_list(this_cpu_ptr(&lazy_list));
 }
 
 /*