powerpc: check_and_cede_processor() never cedes

Commit f948501b ("Make hard_irq_disable() actually hard-disable
interrupts") caused check_and_cede_processor to stop working.
->irq_happened will never be zero right after a hard_irq_disable
so the compiler removes the call to cede_processor completely.

The bug was introduced back in the lazy interrupt handling rework
of 3.4 but was hidden until recently because hard_irq_disable did
nothing.

This issue will eventually appear in 3.4 stable since the
hard_irq_disable fix is marked stable, so mark this one for stable
too.
Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/hw_irq.h

@@ -103,6 +103,11 @@ static inline void hard_irq_disable(void)
 /* include/linux/interrupt.h needs hard_irq_disable to be a macro */
 #define hard_irq_disable	hard_irq_disable
 
+static inline bool lazy_irq_pending(void)
+{
+	return !!(get_paca()->irq_happened & ~PACA_IRQ_HARD_DIS);
+}
+
 /*
  * This is called by asynchronous interrupts to conditionally
  * re-enable hard interrupts when soft-disabled after having

arch/powerpc/platforms/pseries/processor_idle.c

@@ -106,7 +106,7 @@ static void check_and_cede_processor(void)
 	 * we first hard disable then check.
 	 */
 	hard_irq_disable();
-	if (get_paca()->irq_happened == 0)
+	if (!lazy_irq_pending())
 		cede_processor();
 }