genirq: Provide disable_hardirq()

For things like netpoll there is a need to disable an interrupt from
atomic context. Currently netpoll uses disable_irq() which will
sleep-wait on threaded handlers and thus forced_irqthreads breaks
things.

Provide disable_hardirq(), which uses synchronize_hardirq() to only wait
for active hardirq handlers; also change synchronize_hardirq() to
return the status of threaded handlers.

This will allow one to try-disable an interrupt from atomic context, or
in case of request_threaded_irq() to only wait for the hardirq part.

Suggested-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Miller <davem@davemloft.net>
Cc: Eyal Perry <eyalpe@mellanox.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Quentin Lambert <lambert.quentin@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Russell King <linux@arm.linux.org.uk>
Link: http://lkml.kernel.org/r/20150205130623.GH5029@twins.programming.kicks-ass.net
[ Fixed typos and such. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/hardirq.h

@@ -9,7 +9,7 @@
 
 
 extern void synchronize_irq(unsigned int irq);
-extern void synchronize_hardirq(unsigned int irq);
+extern bool synchronize_hardirq(unsigned int irq);
 
 #if defined(CONFIG_TINY_RCU)
 

include/linux/interrupt.h

@@ -184,6 +184,7 @@ extern void devm_free_irq(struct device *dev, unsigned int irq, void *dev_id);
 #endif
 
 extern void disable_irq_nosync(unsigned int irq);
+extern bool disable_hardirq(unsigned int irq);
 extern void disable_irq(unsigned int irq);
 extern void disable_percpu_irq(unsigned int irq);
 extern void enable_irq(unsigned int irq);

kernel/irq/manage.c

@@ -68,14 +68,20 @@ static void __synchronize_hardirq(struct irq_desc *desc)
  *	Do not use this for shutdown scenarios where you must be sure
  *	that all parts (hardirq and threaded handler) have completed.
  *
+ *	Returns: false if a threaded handler is active.
+ *
  *	This function may be called - with care - from IRQ context.
  */
-void synchronize_hardirq(unsigned int irq)
+bool synchronize_hardirq(unsigned int irq)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
 
-	if (desc)
+	if (desc) {
 		__synchronize_hardirq(desc);
+		return !atomic_read(&desc->threads_active);
+	}
+
+	return true;
 }
 EXPORT_SYMBOL(synchronize_hardirq);
 
@@ -440,6 +446,32 @@ void disable_irq(unsigned int irq)
 }
 EXPORT_SYMBOL(disable_irq);
 
+/**
+ *	disable_hardirq - disables an irq and waits for hardirq completion
+ *	@irq: Interrupt to disable
+ *
+ *	Disable the selected interrupt line.  Enables and Disables are
+ *	nested.
+ *	This function waits for any pending hard IRQ handlers for this
+ *	interrupt to complete before returning. If you use this function while
+ *	holding a resource the hard IRQ handler may need you will deadlock.
+ *
+ *	When used to optimistically disable an interrupt from atomic context
+ *	the return value must be checked.
+ *
+ *	Returns: false if a threaded handler is active.
+ *
+ *	This function may be called - with care - from IRQ context.
+ */
+bool disable_hardirq(unsigned int irq)
+{
+	if (!__disable_irq_nosync(irq))
+		return synchronize_hardirq(irq);
+
+	return false;
+}
+EXPORT_SYMBOL_GPL(disable_hardirq);
+
 void __enable_irq(struct irq_desc *desc, unsigned int irq)
 {
 	switch (desc->depth) {