Added the missing memory barrier so ARMv7-A SMP schedulers no longer miss preemptions (#704)

_tx_thread_schedule stores the newly selected thread into
_tx_thread_current_ptr[core] and then reloads _tx_thread_execute_ptr[core]
to detect a concurrent scheduling decision made by another core. On the
other side, _tx_thread_smp_core_interrupt stores the execute pointer and
then reads the current pointer to decide whether an inter-core interrupt is
required. This is a store-buffer pattern: without a barrier both cores can
observe stale values, the interrupt is skipped, and a ready thread with the
highest priority is never scheduled.

The barrier was added to the ARMv8-A SMP scheduler in 6.2.1, but the
ARMv7-A SMP ports were left untouched even though they implement the same
protocol. This adds the corresponding DMB to the Cortex-A5, Cortex-A7 and
Cortex-A9 SMP schedulers for both the AC5 and GNU toolchains.

The GNU variants were verified by assembling them with arm-none-eabi-gcc
13.2.1 for their respective cores.

Refs #209

Assisted-by: Copilot (Opus 5) <noreply@github.com>

ports_smp/cortex_a5_smp/ac5/src/tx_thread_schedule.s

@@ -1,5 +1,6 @@
 ;/***************************************************************************
 ; * Copyright (c) 2024 Microsoft Corporation
+; * Copyright (c) 2026 Eclipse ThreadX contributors
 ; *
 ; * This program and the accompanying materials are made available under the
 ; * terms of the MIT License which is available at
@@ -8,6 +9,8 @@
 ; * SPDX-License-Identifier: MIT
 ; **************************************************************************/
 ;
+; Some portions generated by Copilot (Opus 5).
+;
 ;
 ;/**************************************************************************/
 ;/**************************************************************************/
@@ -164,6 +167,7 @@ _tx_thread_ready_for_execution
     LDR     r2, =_tx_thread_current_ptr         ; Pickup address of current thread
     ADD     r2, r2, r12                         ; Build index into the current thread array
     STR     r0, [r2, #0]                        ; Setup current thread pointer
+    DMB                                         ; Ensure the current thread pointer write completes
 ;
 ;    /* In the time between reading the execute pointer and assigning
 ;       it to the current pointer, the execute pointer was changed by

ports_smp/cortex_a5_smp/gnu/src/tx_thread_schedule.S

@@ -1,5 +1,6 @@
 @/***************************************************************************
 @ * Copyright (c) 2024 Microsoft Corporation
+@ * Copyright (c) 2026 Eclipse ThreadX contributors
 @ *
 @ * This program and the accompanying materials are made available under the
 @ * terms of the MIT License which is available at
@@ -8,6 +9,8 @@
 @ * SPDX-License-Identifier: MIT
 @ **************************************************************************/
 @
+@ Some portions generated by Copilot (Opus 5).
+@
 @
 @/**************************************************************************/
 @/**************************************************************************/
@@ -166,6 +169,7 @@ _tx_thread_ready_for_execution:
     LDR     r2, =_tx_thread_current_ptr         @ Pickup address of current thread
     ADD     r2, r2, r12                         @ Build index into the current thread array
     STR     r0, [r2, #0]                        @ Setup current thread pointer
+    DMB                                         @ Ensure the current thread pointer write completes
 @
 @    /* In the time between reading the execute pointer and assigning
 @       it to the current pointer, the execute pointer was changed by

ports_smp/cortex_a7_smp/ac5/src/tx_thread_schedule.s

@@ -1,5 +1,6 @@
 ;/***************************************************************************
 ; * Copyright (c) 2024 Microsoft Corporation
+; * Copyright (c) 2026 Eclipse ThreadX contributors
 ; *
 ; * This program and the accompanying materials are made available under the
 ; * terms of the MIT License which is available at
@@ -8,6 +9,8 @@
 ; * SPDX-License-Identifier: MIT
 ; **************************************************************************/
 ;
+; Some portions generated by Copilot (Opus 5).
+;
 ;
 ;/**************************************************************************/
 ;/**************************************************************************/
@@ -163,6 +166,7 @@ _tx_thread_ready_for_execution
     LDR     r2, =_tx_thread_current_ptr         ; Pickup address of current thread
     ADD     r2, r2, r12                         ; Build index into the current thread array
     STR     r0, [r2, #0]                        ; Setup current thread pointer
+    DMB                                         ; Ensure the current thread pointer write completes
 ;
 ;    /* In the time between reading the execute pointer and assigning
 ;       it to the current pointer, the execute pointer was changed by

ports_smp/cortex_a7_smp/gnu/src/tx_thread_schedule.S

@@ -1,5 +1,6 @@
 @/***************************************************************************
 @ * Copyright (c) 2024 Microsoft Corporation
+@ * Copyright (c) 2026 Eclipse ThreadX contributors
 @ *
 @ * This program and the accompanying materials are made available under the
 @ * terms of the MIT License which is available at
@@ -8,6 +9,8 @@
 @ * SPDX-License-Identifier: MIT
 @ **************************************************************************/
 @
+@ Some portions generated by Copilot (Opus 5).
+@
 @
 @/**************************************************************************/
 @/**************************************************************************/
@@ -180,6 +183,7 @@ _tx_thread_ready_for_execution:
     LDR     r2, =_tx_thread_current_ptr         @ Pickup address of current thread
     ADD     r2, r2, r12                         @ Build index into the current thread array
     STR     r0, [r2, #0]                        @ Setup current thread pointer
+    DMB                                         @ Ensure the current thread pointer write completes
 @
 @    /* In the time between reading the execute pointer and assigning
 @       it to the current pointer, the execute pointer was changed by

ports_smp/cortex_a9_smp/ac5/src/tx_thread_schedule.s

@@ -1,5 +1,6 @@
 ;/***************************************************************************
 ; * Copyright (c) 2024 Microsoft Corporation
+; * Copyright (c) 2026 Eclipse ThreadX contributors
 ; *
 ; * This program and the accompanying materials are made available under the
 ; * terms of the MIT License which is available at
@@ -8,6 +9,8 @@
 ; * SPDX-License-Identifier: MIT
 ; **************************************************************************/
 ;
+; Some portions generated by Copilot (Opus 5).
+;
 ;
 ;/**************************************************************************/
 ;/**************************************************************************/
@@ -163,6 +166,7 @@ _tx_thread_ready_for_execution
     LDR     r2, =_tx_thread_current_ptr         ; Pickup address of current thread
     ADD     r2, r2, r12                         ; Build index into the current thread array
     STR     r0, [r2, #0]                        ; Setup current thread pointer
+    DMB                                         ; Ensure the current thread pointer write completes
 ;
 ;    /* In the time between reading the execute pointer and assigning
 ;       it to the current pointer, the execute pointer was changed by

ports_smp/cortex_a9_smp/gnu/src/tx_thread_schedule.S

@@ -1,5 +1,6 @@
 @/***************************************************************************
 @ * Copyright (c) 2024 Microsoft Corporation
+@ * Copyright (c) 2026 Eclipse ThreadX contributors
 @ *
 @ * This program and the accompanying materials are made available under the
 @ * terms of the MIT License which is available at
@@ -8,6 +9,8 @@
 @ * SPDX-License-Identifier: MIT
 @ **************************************************************************/
 @
+@ Some portions generated by Copilot (Opus 5).
+@
 @
 @/**************************************************************************/
 @/**************************************************************************/
@@ -165,6 +168,7 @@ _tx_thread_ready_for_execution:
     LDR     r2, =_tx_thread_current_ptr         @ Pickup address of current thread
     ADD     r2, r2, r12                         @ Build index into the current thread array
     STR     r0, [r2, #0]                        @ Setup current thread pointer
+    DMB                                         @ Ensure the current thread pointer write completes
 @
 @    /* In the time between reading the execute pointer and assigning
 @       it to the current pointer, the execute pointer was changed by