[PATCH 3.18-rc3 v9 2/5] irqchip: gic: Make gic_raise_softirq() FIQ-safe

[Daniel Thompson]

It is currently possible for FIQ handlers to re-enter gic_raise_softirq()
and lock up.

    	gic_raise_softirq()
	   lock(x);
---> FIQ
        handle_fiq()
	   gic_raise_softirq()
	      lock(x);		<-- Lockup

Calling printk() from a FIQ handler can trigger this problem because
printk() raises an IPI when it needs to wake_up_klogd(). More generally,
IPIs are the only means for FIQ handlers to safely defer work to less
restrictive calling context so the function to raise them really needs
to be FIQ-safe.

This patch fixes the problem by converting the cpu_map_migration_lock
into a rwlock making it safe to re-enter the function.

Having made it safe to re-enter gic_raise_softirq() we no longer need to
mask interrupts during gic_raise_softirq() because the b.L migration is
always performed from task context.

Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Jason Cooper <jason at lakedaemon.net>
Cc: Russell King <linux at arm.linux.org.uk>
Cc: Marc Zyngier <marc.zyngier at arm.com>
---
 drivers/irqchip/irq-gic.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
index bb4bc20573ea..a53aa11e4f17 100644
--- a/drivers/irqchip/irq-gic.c
+++ b/drivers/irqchip/irq-gic.c
@@ -75,8 +75,11 @@ static DEFINE_RAW_SPINLOCK(irq_controller_lock);
 /*
  * This lock is used by the big.LITTLE migration code to ensure no
  * IPIs can be pended on the old core after the map has been updated.
+ *
+ * This lock may be locked for reading from FIQ handlers and therefore
+ * must not be locked for writing when FIQs are enabled.
  */
-static DEFINE_RAW_SPINLOCK(cpu_map_migration_lock);
+static DEFINE_RWLOCK(cpu_map_migration_lock);
 
 /*
  * The GIC mapping of CPU interfaces does not necessarily match
@@ -625,12 +628,20 @@ static void __init gic_pm_init(struct gic_chip_data *gic)
 #endif
 
 #ifdef CONFIG_SMP
+/*
+ * Raise the specified IPI on all cpus set in mask.
+ *
+ * This function is safe to call from all calling contexts, including
+ * FIQ handlers. It relies on read locks being multiply acquirable to
+ * avoid deadlocks when the function is re-entered at different
+ * exception levels.
+ */
 static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
 {
 	int cpu;
-	unsigned long flags, map = 0;
+	unsigned long map = 0;
 
-	raw_spin_lock_irqsave(&cpu_map_migration_lock, flags);
+	read_lock(&cpu_map_migration_lock);
 
 	/* Convert our logical CPU mask into a physical one. */
 	for_each_cpu(cpu, mask)
@@ -645,7 +656,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
 	/* this always happens on GIC0 */
 	writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
 
-	raw_spin_unlock_irqrestore(&cpu_map_migration_lock, flags);
+	read_unlock(&cpu_map_migration_lock);
 }
 #endif
 
@@ -693,7 +704,8 @@ int gic_get_cpu_id(unsigned int cpu)
  * Migrate all peripheral interrupts with a target matching the current CPU
  * to the interface corresponding to @new_cpu_id.  The CPU interface mapping
  * is also updated.  Targets to other CPU interfaces are unchanged.
- * This must be called with IRQs locally disabled.
+ * This must be called from a task context and with IRQ and FIQ locally
+ * disabled.
  */
 void gic_migrate_target(unsigned int new_cpu_id)
 {
@@ -724,9 +736,9 @@ void gic_migrate_target(unsigned int new_cpu_id)
 	 * pending on the old cpu static. That means we can defer the
 	 * migration until after we have released the irq_controller_lock.
 	 */
-	raw_spin_lock(&cpu_map_migration_lock);
+	write_lock(&cpu_map_migration_lock);
 	gic_cpu_map[cpu] = 1 << new_cpu_id;
-	raw_spin_unlock(&cpu_map_migration_lock);
+	write_unlock(&cpu_map_migration_lock);
 
 	/*
 	 * Find all the peripheral interrupts targetting the current
-- 
1.9.3