Fix stepper/planner block handling, race conditions (#11098)

- Allow planner to alter the deceleration phase of the currently executing block.
- Remove BUSY flag, as it is NON ATOMIC to set bits in the Stepper ISR and Planner at the same time.

Marlin/src/module/stepper.cpp

@@ -107,8 +107,6 @@ Stepper stepper; // Singleton
 
 // public:
 
-block_t* Stepper::current_block = NULL;  // A pointer to the block currently being traced
-
 #if ENABLED(X_DUAL_ENDSTOPS) || ENABLED(Y_DUAL_ENDSTOPS) || ENABLED(Z_DUAL_ENDSTOPS)
   bool Stepper::homing_dual_axis = false;
 #endif
@@ -119,6 +117,8 @@ block_t* Stepper::current_block = NULL;  // A pointer to the block currently bei
 
 // private:
 
+block_t* Stepper::current_block = NULL; // A pointer to the block currently being traced
+
 uint8_t Stepper::last_direction_bits = 0,
         Stepper::axis_did_move;
 
@@ -1665,6 +1665,7 @@ uint32_t Stepper::stepper_block_phase_isr() {
       acceleration_time = deceleration_time = 0;
 
       uint8_t oversampling = 0;                         // Assume we won't use it
+
       #if ENABLED(ADAPTIVE_STEP_SMOOTHING)
         // At this point, we must decide if we can use Stepper movement axis smoothing.
         uint32_t max_rate = current_block->nominal_rate;  // Get the maximum rate (maximum event speed)
@@ -1874,6 +1875,34 @@ uint32_t Stepper::stepper_block_phase_isr() {
   }
 #endif // LIN_ADVANCE
 
+// Check if the given block is busy or not - Must not be called from ISR contexts
+// The current_block could change in the middle of the read by an Stepper ISR, so
+// we must explicitly prevent that!
+bool Stepper::is_block_busy(const block_t* const block) {
+  #ifdef __AVR__
+    // A SW memory barrier, to ensure GCC does not overoptimize loops
+    #define sw_barrier() asm volatile("": : :"memory");
+
+    // Keep reading until 2 consecutive reads return the same value,
+    // meaning there was no update in-between caused by an interrupt.
+    // This works because stepper ISRs happen at a slower rate than
+    // successive reads of a variable, so 2 consecutive reads with
+    // the same value means no interrupt updated it.
+    block_t* vold, *vnew = current_block;
+    sw_barrier();
+    do {
+      vold = vnew;
+      vnew = current_block;
+      sw_barrier();
+    } while (vold != vnew);
+  #else
+    block_t *vnew = current_block;
+  #endif
+
+  // Return if the block is busy or not
+  return block == vnew;
+}
+
 void Stepper::init() {
 
   // Init Digipot Motor Current