[RFC PATCH 8/9] powerpc/64s: interrupt soft-enable race fix

[Nicholas Piggin <npiggin@gmail.com>]

Prevent interrupt restore from allowing racing hard interrupts going
ahead of previous soft-pending ones, by using the soft-masked restart
handler to allow a store to clear the soft-mask while knowing nothing
is soft-pending.

This probably doesn't matter much in practice, but it's a simple
demonstrator / test case to exercise the restart table logic.

XXX: 64e can't do this unless it adds restart table support

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/irq.c | 79 +++++++++++++++++++++++++--------------
 1 file changed, 50 insertions(+), 29 deletions(-)

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index c8185f709d26..9671468b2c51 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -317,52 +317,73 @@ notrace void arch_local_irq_restore(unsigned long mask)
 	unsigned char irq_happened;
 
 	/* Write the new soft-enabled value */
-	irq_soft_mask_set(mask);
-	if (mask)
+	if (mask) {
+		irq_soft_mask_set(mask);
 		return;
+	}
 
 	/*
-	 * From this point onward, we can take interrupts, preempt,
-	 * etc... unless we got hard-disabled. We check if an event
-	 * happened. If none happened, we know we can just return.
-	 *
-	 * We may have preempted before the check below, in which case
-	 * we are checking the "new" CPU instead of the old one. This
-	 * is only a problem if an event happened on the "old" CPU.
+	 * After the stb, interrupts are unmasked and there are no interrupts
+	 * pending replay. The restart sequence makes this atomic with
+	 * respect to soft-masked interrupts. If this was just a simple code
+	 * sequence, a soft-masked interrupt could become pending right after
+	 * the comparison and before the stb.
 	 *
-	 * External interrupt events will have caused interrupts to
-	 * be hard-disabled, so there is no problem, we
-	 * cannot have preempted.
+	 * This allows interrupts to be unmasked without hard disabling, and
+	 * also without new hard interrupts coming in ahead of pending ones.
 	 */
+	asm_volatile_goto(
+"1:					\n"
+"		lbz	9,%0(13)	\n"
+"		cmpwi	9,0		\n"
+"		bne	%l[happened]	\n"
+"		stb	9,%1(13)	\n"
+"2:					\n"
+		RESTART_TABLE(1b, 2b, 1b)
+	: : "i" (offsetof(struct paca_struct, irq_happened)),
+	    "i" (offsetof(struct paca_struct, irq_soft_mask))
+	: "cr0", "r9"
+	: happened);
+
+	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
+		WARN_ON_ONCE(!(mfmsr() & MSR_EE));
+
+	return;
+
+happened:
 	irq_happened = get_irq_happened();
-	if (!irq_happened) {
+	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
+		WARN_ON_ONCE(!irq_happened);
+
+	if (irq_happened == PACA_IRQ_HARD_DIS) {
 		if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
-			WARN_ON_ONCE(!(mfmsr() & MSR_EE));
+			WARN_ON_ONCE(mfmsr() & MSR_EE);
+		irq_soft_mask_set(IRQS_ENABLED);
+		local_paca->irq_happened = 0;
+		__hard_irq_enable();
 		return;
 	}
 
-	/* We need to hard disable to replay. */
+	/* Have interrupts to replay, need to hard disable first */
 	if (!(irq_happened & PACA_IRQ_HARD_DIS)) {
-		if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
-			WARN_ON_ONCE(!(mfmsr() & MSR_EE));
+		if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG)) {
+			if (!(mfmsr() & MSR_EE)) {
+				/*
+				 * An interrupt could have come in and cleared
+				 * MSR[EE] and set IRQ_HARD_DIS, so check
+				 * IRQ_HARD_DIS again and warn if it is still
+				 * clear.
+				 */
+				irq_happened = get_irq_happened();
+				WARN_ON_ONCE(!(irq_happened & PACA_IRQ_HARD_DIS));
+			}
+		}
 		__hard_irq_disable();
 	} else {
-		/*
-		 * We should already be hard disabled here. We had bugs
-		 * where that wasn't the case so let's dbl check it and
-		 * warn if we are wrong. Only do that when IRQ tracing
-		 * is enabled as mfmsr() can be costly.
-		 */
 		if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG)) {
 			if (WARN_ON_ONCE(mfmsr() & MSR_EE))
 				__hard_irq_disable();
 		}
-
-		if (irq_happened == PACA_IRQ_HARD_DIS) {
-			local_paca->irq_happened = 0;
-			__hard_irq_enable();
-			return;
-		}
 	}
 
 	/*
-- 
2.23.0