Add comment regarding LDREX, STREX and exception behavior

teensy4/EventResponder.cpp

@@ -348,6 +348,13 @@ extern "C" void systick_isr(void)
 	systick_millis_count++;
 }
 
+// Entry to any ARM exception clears the LDREX exclusive access flag.
+// So we do not need to do anything with "systick_safe_read" here, as
+// this code depends on this Cortex-M7 hardware feature to cause any
+// STREX instruction to return 1 (fail status) after returning to
+// main program or lower priority interrupts.
+//  https://developer.arm.com/documentation/dui0646/c/the-cortex-m7-processor/memory-model/synchronization-primitives
+
 extern "C" void systick_isr_with_timer_events(void)
 {
 	systick_cycle_count = ARM_DWT_CYCCNT;