From 271ced73413ebf8d662cce2a582ac9b795104a8c Mon Sep 17 00:00:00 2001
From: Sebastianv650 <sebastian_v650@kabelmail.de>
Date: Sun, 12 Feb 2017 18:00:14 +0100
Subject: [PATCH] Prevent re-entering of temperature ISR

If Marlin is inside the temperature ISR, the stepper ISR is enabled. If
a stepper event is now happening Marlin will proceed with the stepper
ISR. Now, at the end of the stepper ISR, the temperatre ISR gets enabled
again. While Marlin proceed the rest of the temperature ISR, it's now
vulnerable to a second ISR call.
---
 Marlin/temperature.cpp | 11 ++++++++++-
 Marlin/temperature.h   |  2 ++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/Marlin/temperature.cpp b/Marlin/temperature.cpp
index 8120e0d6c5..d7462bc99d 100644
--- a/Marlin/temperature.cpp
+++ b/Marlin/temperature.cpp
@@ -1483,8 +1483,15 @@ void Temperature::set_current_temp_raw() {
  */
 ISR(TIMER0_COMPB_vect) { Temperature::isr(); }
 
+volatile bool Temperature::in_temp_isr = false;
+
 void Temperature::isr() {
-  //Allow UART and stepper ISRs
+  // The stepper ISR can interrupt this ISR. When it does it re-enables this ISR
+  // at the end of its run, potentially causing re-entry. This flag prevents it.
+  if (in_temp_isr) return;
+  in_temp_isr = true;
+  
+  // Allow UART and stepper ISRs
   CBI(TIMSK0, OCIE0B); //Disable Temperature ISR
   sei();
 
@@ -1949,5 +1956,7 @@ void Temperature::isr() {
     }
   #endif
 
+  cli();
+  in_temp_isr = false;
   SBI(TIMSK0, OCIE0B); //re-enable Temperature ISR
 }
diff --git a/Marlin/temperature.h b/Marlin/temperature.h
index 182efd5645..e592148447 100644
--- a/Marlin/temperature.h
+++ b/Marlin/temperature.h
@@ -61,6 +61,8 @@ class Temperature {
                  current_temperature_bed_raw,
                  target_temperature_bed;
 
+    static volatile bool in_temp_isr;
+
     #if ENABLED(TEMP_SENSOR_1_AS_REDUNDANT)
       static float redundant_temperature;
     #endif