[PATCH 5.8 209/255] x86/hotplug: Silence APIC only after all interrupts are migrated

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Ashok Raj <ashok.raj@xxxxxxxxx>

commit 52d6b926aabc47643cd910c85edb262b7f44c168 upstream.

There is a race when taking a CPU offline. Current code looks like this:

native_cpu_disable()
{
	...
	apic_soft_disable();
	/*
	 * Any existing set bits for pending interrupt to
	 * this CPU are preserved and will be sent via IPI
	 * to another CPU by fixup_irqs().
	 */
	cpu_disable_common();
	{
		....
		/*
		 * Race window happens here. Once local APIC has been
		 * disabled any new interrupts from the device to
		 * the old CPU are lost
		 */
		fixup_irqs(); // Too late to capture anything in IRR.
		...
	}
}

The fix is to disable the APIC *after* cpu_disable_common().

Testing was done with a USB NIC that provided a source of frequent
interrupts. A script migrated interrupts to a specific CPU and
then took that CPU offline.

Fixes: 60dcaad5736f ("x86/hotplug: Silence APIC and NMI when CPU is dead")
Reported-by: Evan Green <evgreen@xxxxxxxxxxxx>
Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
Tested-by: Evan Green <evgreen@xxxxxxxxxxxx>
Reviewed-by: Evan Green <evgreen@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/lkml/875zdarr4h.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/r/1598501530-45821-1-git-send-email-ashok.raj@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kernel/smpboot.c |   26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1604,14 +1604,28 @@ int native_cpu_disable(void)
 	if (ret)
 		return ret;
 
-	/*
-	 * Disable the local APIC. Otherwise IPI broadcasts will reach
-	 * it. It still responds normally to INIT, NMI, SMI, and SIPI
-	 * messages.
-	 */
-	apic_soft_disable();
 	cpu_disable_common();
 
+        /*
+         * Disable the local APIC. Otherwise IPI broadcasts will reach
+         * it. It still responds normally to INIT, NMI, SMI, and SIPI
+         * messages.
+         *
+         * Disabling the APIC must happen after cpu_disable_common()
+         * which invokes fixup_irqs().
+         *
+         * Disabling the APIC preserves already set bits in IRR, but
+         * an interrupt arriving after disabling the local APIC does not
+         * set the corresponding IRR bit.
+         *
+         * fixup_irqs() scans IRR for set bits so it can raise a not
+         * yet handled interrupt on the new destination CPU via an IPI
+         * but obviously it can't do so for IRR bits which are not set.
+         * IOW, interrupts arriving after disabling the local APIC will
+         * be lost.
+         */
+	apic_soft_disable();
+
 	return 0;
 }