Fix race condition and message loss in Cortex-M IAR ports (#516)

- Added compiler memory barriers to BASEPRI management functions in tx_port.h.
- Added architectural barriers (DSB/ISB) to scheduler return paths in tx_port.h and tx_thread_system_return.s to prevent fall-through before context switch.
- These changes address spurious thread resumption and lost messages, especially when TX_NOT_INTERRUPTABLE is enabled.

Assisted-by: Gemini (Gemini 2.0 Flash)

Author: fdesbiens

ports/cortex_m33/iar/inc/tx_port.h

@@ -9,6 +9,8 @@
  * SPDX-License-Identifier: MIT
  **************************************************************************/
 
+// Some portions generated by Gemini (Gemini 2.0 Flash).
+
 
 /**************************************************************************/
 /**************************************************************************/
@@ -541,7 +543,7 @@ UINT posture;
 #ifdef TX_PORT_USE_BASEPRI
 __attribute__( ( always_inline ) ) static inline void __set_basepri_value(UINT basepri_value)
 {
-    __asm__ volatile ("MSR  BASEPRI,%0 ": : "r" (basepri_value));
+    __asm__ volatile ("MSR  BASEPRI,%0 ": : "r" (basepri_value) : "memory");
 }
 #else
 __attribute__( ( always_inline ) ) static inline void __enable_interrupts(void)
@@ -554,6 +556,7 @@ __attribute__( ( always_inline ) ) static inline void __restore_interrupt(UINT i
 {
 #ifdef TX_PORT_USE_BASEPRI
     __set_basepri_value(int_posture);
+    __asm__ volatile ("" : : : "memory");
 #else
     __asm__ volatile ("MSR  PRIMASK,%0": : "r" (int_posture): "memory");
 #endif
@@ -579,6 +582,7 @@ UINT interrupt_save;
 
     /* Set PendSV to invoke ThreadX scheduler.  */
     *((volatile ULONG *) 0xE000ED04) = ((ULONG) 0x10000000);
+    __asm__ volatile ("dsb 0xF \n isb 0xF " : : : "memory");
     if (_tx_ipsr_get() == 0)
     {
         interrupt_save = __get_interrupt_posture();

ports/cortex_m33/iar/src/tx_thread_system_return.s

@@ -1,16 +1,19 @@
-/***************************************************************************
- * Copyright (c) 2024 Microsoft Corporation
- * Copyright (c) 2026-present Eclipse ThreadX contributors
- *
- * This program and the accompanying materials are made available under the
- * terms of the MIT License which is available at
- * https://opensource.org/licenses/MIT.
- *
- * SPDX-License-Identifier: MIT
- **************************************************************************/
+;***************************************************************************
+;* Copyright (c) 2024 Microsoft Corporation
+;* Copyright (c) 2026-present Eclipse ThreadX contributors
+;*
+;* This program and the accompanying materials are made available under the
+;* terms of the MIT License which is available at
+;* https://opensource.org/licenses/MIT.
+;*
+;* SPDX-License-Identifier: MIT
+;**************************************************************************
 
+; Some portions generated by Gemini (Gemini 2.0 Flash).
+
+
+;**************************************************************************
 
-/**************************************************************************/
 /**************************************************************************/
 /**                                                                       */
 /** ThreadX Component                                                     */
@@ -69,6 +72,8 @@ _tx_thread_system_return:
     MOV     r0, #0x10000000                         // Load PENDSVSET bit
     MOV     r1, #0xE000E000                         // Load NVIC base
     STR     r0, [r1, #0xD04]                        // Set PENDSVBIT in ICSR
+    DSB     SY                                      // Ensure memory access is complete
+    ISB     SY                                      // Flush pipeline
     MRS     r0, IPSR                                // Pickup IPSR
     CMP     r0, #0                                  // Is it a thread returning?
     BNE     _isr_context                            // If ISR, skip interrupt enable