Subject: softirq: Check preemption after reenabling interrupts
From: Thomas Gleixner <tglx@linutronix.de>
Date: Sun, 13 Nov 2011 17:17:09 +0100 (CET)

raise_softirq_irqoff() disables interrupts and wakes the softirq
daemon, but after reenabling interrupts there is no preemption check,
so the execution of the softirq thread might be delayed arbitrarily.

In principle we could add that check to local_irq_enable/restore, but
that's overkill as the rasie_softirq_irqoff() sections are the only
ones which show this behaviour.

Reported-by: Carsten Emde <cbe@osadl.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org
---
 block/blk-iopoll.c      |    3 +++
 block/blk-softirq.c     |    3 +++
 include/linux/preempt.h |    3 +++
 net/core/dev.c          |    6 ++++++
 4 files changed, 15 insertions(+)

Index: linux-stable/block/blk-iopoll.c
===================================================================
--- linux-stable.orig/block/blk-iopoll.c
+++ linux-stable/block/blk-iopoll.c
@@ -38,6 +38,7 @@ void blk_iopoll_sched(struct blk_iopoll 
 	list_add_tail(&iop->list, &__get_cpu_var(blk_cpu_iopoll));
 	__raise_softirq_irqoff(BLOCK_IOPOLL_SOFTIRQ);
 	local_irq_restore(flags);
+	preempt_check_resched_rt();
 }
 EXPORT_SYMBOL(blk_iopoll_sched);
 
@@ -135,6 +136,7 @@ static void blk_iopoll_softirq(struct so
 		__raise_softirq_irqoff(BLOCK_IOPOLL_SOFTIRQ);
 
 	local_irq_enable();
+	preempt_check_resched_rt();
 }
 
 /**
@@ -204,6 +206,7 @@ static int __cpuinit blk_iopoll_cpu_noti
 				 &__get_cpu_var(blk_cpu_iopoll));
 		__raise_softirq_irqoff(BLOCK_IOPOLL_SOFTIRQ);
 		local_irq_enable();
+		preempt_check_resched_rt();
 	}
 
 	return NOTIFY_OK;
Index: linux-stable/block/blk-softirq.c
===================================================================
--- linux-stable.orig/block/blk-softirq.c
+++ linux-stable/block/blk-softirq.c
@@ -51,6 +51,7 @@ static void trigger_softirq(void *data)
 		raise_softirq_irqoff(BLOCK_SOFTIRQ);
 
 	local_irq_restore(flags);
+	preempt_check_resched_rt();
 }
 
 /*
@@ -93,6 +94,7 @@ static int __cpuinit blk_cpu_notify(stru
 				 &__get_cpu_var(blk_cpu_done));
 		raise_softirq_irqoff(BLOCK_SOFTIRQ);
 		local_irq_enable();
+		preempt_check_resched_rt();
 	}
 
 	return NOTIFY_OK;
@@ -150,6 +152,7 @@ do_local:
 		goto do_local;
 
 	local_irq_restore(flags);
+	preempt_check_resched_rt();
 }
 
 /**
Index: linux-stable/include/linux/preempt.h
===================================================================
--- linux-stable.orig/include/linux/preempt.h
+++ linux-stable/include/linux/preempt.h
@@ -56,8 +56,10 @@ do { \
 
 #ifndef CONFIG_PREEMPT_RT_BASE
 # define preempt_enable_no_resched()	sched_preempt_enable_no_resched()
+# define preempt_check_resched_rt()	do { } while (0)
 #else
 # define preempt_enable_no_resched()	preempt_enable()
+# define preempt_check_resched_rt()	preempt_check_resched()
 #endif
 
 #define preempt_enable() \
@@ -105,6 +107,7 @@ do { \
 #define preempt_disable_notrace()		do { } while (0)
 #define preempt_enable_no_resched_notrace()	do { } while (0)
 #define preempt_enable_notrace()		do { } while (0)
+#define preempt_check_resched_rt()	do { } while (0)
 
 #endif /* CONFIG_PREEMPT_COUNT */
 
Index: linux-stable/net/core/dev.c
===================================================================
--- linux-stable.orig/net/core/dev.c
+++ linux-stable/net/core/dev.c
@@ -1836,6 +1836,7 @@ static inline void __netif_reschedule(st
 	sd->output_queue_tailp = &q->next_sched;
 	raise_softirq_irqoff(NET_TX_SOFTIRQ);
 	local_irq_restore(flags);
+	preempt_check_resched_rt();
 }
 
 void __netif_schedule(struct Qdisc *q)
@@ -1857,6 +1858,7 @@ void dev_kfree_skb_irq(struct sk_buff *s
 		sd->completion_queue = skb;
 		raise_softirq_irqoff(NET_TX_SOFTIRQ);
 		local_irq_restore(flags);
+		preempt_check_resched_rt();
 	}
 }
 EXPORT_SYMBOL(dev_kfree_skb_irq);
@@ -2927,6 +2929,7 @@ enqueue:
 	rps_unlock(sd);
 
 	local_irq_restore(flags);
+	preempt_check_resched_rt();
 
 	atomic_long_inc(&skb->dev->rx_dropped);
 	kfree_skb(skb);
@@ -3790,6 +3793,7 @@ static void net_rps_action_and_irq_enabl
 	} else
 #endif
 		local_irq_enable();
+	preempt_check_resched_rt();
 }
 
 static int process_backlog(struct napi_struct *napi, int quota)
@@ -3862,6 +3866,7 @@ void __napi_schedule(struct napi_struct 
 	local_irq_save(flags);
 	____napi_schedule(&__get_cpu_var(softnet_data), n);
 	local_irq_restore(flags);
+	preempt_check_resched_rt();
 }
 EXPORT_SYMBOL(__napi_schedule);
 
@@ -6367,6 +6372,7 @@ static int dev_cpu_callback(struct notif
 
 	raise_softirq_irqoff(NET_TX_SOFTIRQ);
 	local_irq_enable();
+	preempt_check_resched_rt();
 
 	/* Process offline CPU's input_pkt_queue */
 	while ((skb = __skb_dequeue(&oldsd->process_queue))) {