Patch "powerpc/64: Fix __check_irq_replay missing decrementer interrupt" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/64: Fix __check_irq_replay missing decrementer interrupt

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-64-fix-__check_irq_replay-missing-decrementer-interrupt.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Nicholas Piggin <[email protected]>
Date: Tue, 1 Aug 2017 23:59:28 +1000
Subject: powerpc/64: Fix __check_irq_replay missing decrementer interrupt

From: Nicholas Piggin <[email protected]>

commit 3db40c312c2c1eb2187c5731102fa8ff380e6e40 upstream.

If the decrementer wraps again and de-asserts the decrementer exception while hard-disabled, __check_irq_replay() has a test to notice the wrap when interrupts are re-enabled.

The decrementer check must be done when clearing the PACA_IRQ_HARD_DIS flag, not when the PACA_IRQ_DEC flag is tested. Previously this worked because the decrementer interrupt was always the first one checked after clearing the hard disable flag, but HMI check was moved ahead of that, which introduced this bug.

This can cause a missed decrementer interrupt if we soft-disable interrupts then take an HMI which is recorded in irq_happened, then hard-disable interrupts for > 4s to wrap the decrementer.

---
 arch/powerpc/kernel/irq.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

@@ -146,6 +146,19 @@ notrace unsigned int __check_irq_replay(
 
 	/* Clear bit 0 which we wouldn't clear otherwise */
 	local_paca->irq_happened &= ~PACA_IRQ_HARD_DIS;
+	if (happened & PACA_IRQ_HARD_DIS) {
+		/*
+		 * We may have missed a decrementer interrupt if hard disabled.
+		 * Check the decrementer register in case we had a rollover
+		 * while hard disabled.
+		 */
+		if (!(happened & PACA_IRQ_DEC)) {
+			if (decrementer_check_overflow()) {
+				local_paca->irq_happened |= PACA_IRQ_DEC;
+				happened |= PACA_IRQ_DEC;
+			}
+		}
+	}
 
 	/*
 	 * Force the delivery of pending soft-disabled interrupts on PS3.

@@ -171,7 +184,7 @@ notrace unsigned int __check_irq_replay(
 	 * in case we also had a rollover while hard disabled
 	 */
 	local_paca->irq_happened &= ~PACA_IRQ_DEC;
-	if ((happened & PACA_IRQ_DEC) || decrementer_check_overflow())
+	if (happened & PACA_IRQ_DEC)
 		return 0x900;
 
 	/* Finally check if an external interrupt happened */

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/powerpc-64-fix-__check_irq_replay-missing-decrementer-interrupt.patch