IRQ affinity notifiers vs RT

[Ben Hutchings]

Alexandra Kossovsky reported the following lockdep splat when testing an
out-of-tree version of sfc on 3.6-rt.  The problem is specific to RT,
and we haven't tested anything later but I think it's still unfixed.

> ======================================================
> [ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]
> 3.6.11.2-rt33.39.el6rt.x86_64.debug #1 Tainted: G           O
> ------------------------------------------------------
> insmod/3076 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
>  (&(&(&gcwq->lock)->lock)->wait_lock){+.+...}, at: [<ffffffff81589a78>] rt_spin_lock_slowlock+0x48/0x2f0
> 
> and this task is already holding:
>  (&irq_desc_lock_class){-.....}, at: [<ffffffff810ec226>] irq_set_affinity+0x46/0x80

irq_set_affinity() holds the irq_desc lock, and then schedules a work
item to call the IRQ affinity notifier.

> which would create a new lock dependency:
>  (&irq_desc_lock_class){-.....} -> (&(&(&gcwq->lock)->lock)->wait_lock){+.+...}
> 
> but this new dependency connects a HARDIRQ-irq-safe lock:
>  (&irq_desc_lock_class){-.....}
> ... which became HARDIRQ-irq-safe at:
>   [<ffffffff810adba2>] mark_irqflags+0x172/0x190
>   [<ffffffff810af2c4>] __lock_acquire+0x344/0x4e0
>   [<ffffffff810af4ea>] lock_acquire+0x8a/0x140
>   [<ffffffff8158ac50>] _raw_spin_lock+0x40/0x80
>   [<ffffffff810edfae>] handle_level_irq+0x1e/0x100
>   [<ffffffff810044e1>] handle_irq+0x71/0x190
>   [<ffffffff815943ad>] do_IRQ+0x5d/0xe0
>   [<ffffffff8158b32c>] ret_from_intr+0x0/0x13
>   [<ffffffff81d16d9d>] tsc_init+0x24/0x102
>   [<ffffffff81d13d77>] x86_late_time_init+0xf/0x11
>   [<ffffffff81d10cfe>] start_kernel+0x312/0x3c6
>   [<ffffffff81d1032d>] x86_64_start_reservations+0x131/0x136
>   [<ffffffff81d1041f>] x86_64_start_kernel+0xed/0xf4

Obviously, irq_desc is used in hard-IRQ context.

> to a HARDIRQ-irq-unsafe lock:
>  (&(&(&gcwq->lock)->lock)->wait_lock){+.+...}
> ... which became HARDIRQ-irq-unsafe at:
> ...  [<ffffffff810adb50>] mark_irqflags+0x120/0x190
>   [<ffffffff810af2c4>] __lock_acquire+0x344/0x4e0
>   [<ffffffff810af4ea>] lock_acquire+0x8a/0x140
>   [<ffffffff8158ac50>] _raw_spin_lock+0x40/0x80
>   [<ffffffff81589a78>] rt_spin_lock_slowlock+0x48/0x2f0
>   [<ffffffff8158a1b6>] rt_spin_lock+0x16/0x40
>   [<ffffffff8106cf99>] create_worker+0x69/0x220
>   [<ffffffff81d2bae5>] init_workqueues+0x24b/0x3f8
>   [<ffffffff810001c2>] do_one_initcall+0x42/0x170
>   [<ffffffff81d10775>] kernel_init+0xe5/0x17a
>   [<ffffffff81593c24>] kernel_thread_helper+0x4/0x10
[...]

Workqueue code uses spin_lock_irq() on the workqueue lock, which with
PREEMPT_RT enabled doesn't actually block IRQs.

In 3.6, the irq_cpu_rmap functions relies on a workqueue flush to finish
any outstanding notifications before freeing the cpu_rmap that they use.
This won't be reliable if the notification is scheduled after releasing
the irq_desc lock.

However, following commit 896f97ea95c1 ('lib: cpu_rmap: avoid flushing
all workqueues') in 3.8, I think that it is sufficient to do only
kref_get(&desc->affinity_notify->kref) in __irq_set_affinity_locked()
and then call schedule_work() in irq_set_affinity() after releasing the
lock.  Something like this (untested):

diff --git a/arch/mips/cavium-octeon/octeon-irq.c b/arch/mips/cavium-octeon/octeon-irq.c
index 9d36774..0406481 100644
--- a/arch/mips/cavium-octeon/octeon-irq.c
+++ b/arch/mips/cavium-octeon/octeon-irq.c
@@ -635,7 +635,8 @@ static void octeon_irq_cpu_offline_ciu(struct irq_data *data)
 		cpumask_clear(&new_affinity);
 		cpumask_set_cpu(cpumask_first(cpu_online_mask), &new_affinity);
 	}
-	__irq_set_affinity_locked(data, &new_affinity);
+	/* XXX No-one else calls this; why does this chip need it? */
+	__irq_set_affinity_locked(data, &new_affinity, NULL);
 }
 
 static int octeon_irq_ciu_set_affinity(struct irq_data *data,
diff --git a/include/linux/irq.h b/include/linux/irq.h
index f04d3ba..de992f4 100644
--- a/include/linux/irq.h
+++ b/include/linux/irq.h
@@ -380,7 +380,9 @@ extern void remove_percpu_irq(unsigned int irq, struct irqaction *act);
 
 extern void irq_cpu_online(void);
 extern void irq_cpu_offline(void);
-extern int __irq_set_affinity_locked(struct irq_data *data,  const struct cpumask *cpumask);
+extern int __irq_set_affinity_locked(struct irq_data *data,
+				     const struct cpumask *cpumask,
+				     struct irq_affinity_notify **notify);
 
 #ifdef CONFIG_GENERIC_HARDIRQS
 
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 514bcfd..157afa2 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -162,12 +162,16 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
 	return ret;
 }
 
-int __irq_set_affinity_locked(struct irq_data *data, const struct cpumask *mask)
+int __irq_set_affinity_locked(struct irq_data *data, const struct cpumask *mask,
+			      struct irq_affinity_notify **notify)
 {
 	struct irq_chip *chip = irq_data_get_irq_chip(data);
 	struct irq_desc *desc = irq_data_to_desc(data);
 	int ret = 0;
 
+	if (notify)
+		*notify = NULL;
+
 	if (!chip || !chip->irq_set_affinity)
 		return -EINVAL;
 
@@ -178,9 +182,9 @@ int __irq_set_affinity_locked(struct irq_data *data, const struct cpumask *mask)
 		irq_copy_pending(desc, mask);
 	}
 
-	if (desc->affinity_notify) {
+	if (notify && desc->affinity_notify) {
 		kref_get(&desc->affinity_notify->kref);
-		schedule_work(&desc->affinity_notify->work);
+		*notify = desc->affinity_notify;
 	}
 	irqd_set(data, IRQD_AFFINITY_SET);
 
@@ -196,6 +200,7 @@ int __irq_set_affinity_locked(struct irq_data *data, const struct cpumask *mask)
 int irq_set_affinity(unsigned int irq, const struct cpumask *mask)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
+	struct irq_affinity_notify *notify;
 	unsigned long flags;
 	int ret;
 
@@ -203,8 +208,13 @@ int irq_set_affinity(unsigned int irq, const struct cpumask *mask)
 		return -EINVAL;
 
 	raw_spin_lock_irqsave(&desc->lock, flags);
-	ret =  __irq_set_affinity_locked(irq_desc_get_irq_data(desc), mask);
+	ret =  __irq_set_affinity_locked(irq_desc_get_irq_data(desc), mask,
+					 &notify);
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
+
+	if (notify)
+		schedule_work(&notify->work);
+
 	return ret;
 }
 
--- END ---

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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