Revert "kernel/sched: Fix free-memory write when ISRs abort _current"

This reverts commit 61c70626a5.

This PR introduced 2 regressions in main CI:
71977 & 71978
Let's revert it by now to get main's CI passing again.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

kernel/include/ksched.h

@@ -37,8 +37,6 @@ BUILD_ASSERT(K_LOWEST_APPLICATION_THREAD_PRIO
 #define Z_ASSERT_VALID_PRIO(prio, entry_point) __ASSERT((prio) == -1, "")
 #endif /* CONFIG_MULTITHREADING */
 
-extern struct k_thread _thread_dummies[CONFIG_MP_MAX_NUM_CPUS];
-
 void z_sched_init(void);
 void z_move_thread_to_end_of_prio_q(struct k_thread *thread);
 void z_unpend_thread_no_timeout(struct k_thread *thread);

kernel/init.c

@@ -650,7 +650,12 @@ FUNC_NORETURN void z_cstart(void)
 	LOG_CORE_INIT();
 
 #if defined(CONFIG_MULTITHREADING)
-	z_dummy_thread_init(&_thread_dummies[0]);
+	/* Note: The z_ready_thread() call in prepare_multithreading() requires
+	 * a dummy thread even if CONFIG_ARCH_HAS_CUSTOM_SWAP_TO_MAIN=y
+	 */
+	struct k_thread dummy_thread;
+
+	z_dummy_thread_init(&dummy_thread);
 #endif /* CONFIG_MULTITHREADING */
 	/* do any necessary initialization of static devices */
 	z_device_state_init();

kernel/sched.c

@@ -30,11 +30,6 @@ extern struct k_thread *pending_current;
 
 struct k_spinlock _sched_spinlock;
 
-/* Storage to "complete" the context switch from an invalid/incomplete thread
- * context (ex: exiting an ISR that aborted _current)
- */
-__incoherent struct k_thread _thread_dummies[CONFIG_MP_MAX_NUM_CPUS];
-
 static void update_cache(int preempt_ok);
 static void halt_thread(struct k_thread *thread, uint8_t new_state);
 static void add_to_waitq_locked(struct k_thread *thread, _wait_q_t *wait_q);
@@ -428,9 +423,8 @@ void z_sched_start(struct k_thread *thread)
  * another CPU to catch the IPI we sent and halt.  Note that we check
  * for ourselves being asynchronously halted first to prevent simple
  * deadlocks (but not complex ones involving cycles of 3+ threads!).
- * Acts to release the provided lock before returning.
  */
-static void thread_halt_spin(struct k_thread *thread, k_spinlock_key_t key)
+static k_spinlock_key_t thread_halt_spin(struct k_thread *thread, k_spinlock_key_t key)
 {
 	if (is_halting(_current)) {
 		halt_thread(_current,
@@ -438,11 +432,10 @@ static void thread_halt_spin(struct k_thread *thread, k_spinlock_key_t key)
 	}
 	k_spin_unlock(&_sched_spinlock, key);
 	while (is_halting(thread)) {
-		unsigned int k = arch_irq_lock();
-
-		arch_spin_relax(); /* Requires interrupts be masked */
-		arch_irq_unlock(k);
 	}
+	key = k_spin_lock(&_sched_spinlock);
+	z_sched_switch_spin(thread);
+	return key;
 }
 
 /* Shared handler for k_thread_{suspend,abort}().  Called with the
@@ -472,7 +465,8 @@ static void z_thread_halt(struct k_thread *thread, k_spinlock_key_t key,
 		arch_sched_ipi();
 #endif
 		if (arch_is_in_isr()) {
-			thread_halt_spin(thread, key);
+			key = thread_halt_spin(thread, key);
+			k_spin_unlock(&_sched_spinlock, key);
 		} else  {
 			add_to_waitq_locked(_current, wq);
 			z_swap(&_sched_spinlock, key);
@@ -486,10 +480,6 @@ static void z_thread_halt(struct k_thread *thread, k_spinlock_key_t key,
 			k_spin_unlock(&_sched_spinlock, key);
 		}
 	}
-	/* NOTE: the scheduler lock has been released.  Don't put
-	 * logic here, it's likely to be racy/deadlocky even if you
-	 * re-take the lock!
-	 */
 }
 
 
@@ -1289,8 +1279,6 @@ extern void thread_abort_hook(struct k_thread *thread);
  */
 static void halt_thread(struct k_thread *thread, uint8_t new_state)
 {
-	bool dummify = false;
-
 	/* We hold the lock, and the thread is known not to be running
 	 * anywhere.
 	 */
@@ -1307,16 +1295,6 @@ static void halt_thread(struct k_thread *thread, uint8_t new_state)
 			}
 			(void)z_abort_thread_timeout(thread);
 			unpend_all(&thread->join_queue);
-
-			/* Edge case: aborting _current from within an
-			 * ISR that preempted it requires clearing the
-			 * _current pointer so the upcoming context
-			 * switch doesn't clobber the now-freed
-			 * memory
-			 */
-			if (thread == _current && arch_is_in_isr()) {
-				dummify = true;
-			}
 		}
 #ifdef CONFIG_SMP
 		unpend_all(&thread->halt_queue);
@@ -1355,16 +1333,6 @@ static void halt_thread(struct k_thread *thread, uint8_t new_state)
 #ifdef CONFIG_THREAD_ABORT_NEED_CLEANUP
 		k_thread_abort_cleanup(thread);
 #endif /* CONFIG_THREAD_ABORT_NEED_CLEANUP */
-
-		/* Do this "set _current to dummy" step last so that
-		 * subsystems above can rely on _current being
-		 * unchanged.  Disabled for posix as that arch
-		 * continues to use the _current pointer in its swap
-		 * code.
-		 */
-		if (dummify && !IS_ENABLED(CONFIG_ARCH_POSIX)) {
-			z_dummy_thread_init(&_thread_dummies[_current_cpu->id]);
-		}
 	}
 }
 

kernel/smp.c

@@ -108,6 +108,7 @@ static void wait_for_start_signal(atomic_t *start_flag)
 
 static inline void smp_init_top(void *arg)
 {
+	struct k_thread dummy_thread;
 	struct cpu_start_cb csc = arg ? *(struct cpu_start_cb *)arg : (struct cpu_start_cb){0};
 
 	/* Let start_cpu() know that this CPU has powered up. */
@@ -122,7 +123,7 @@ static inline void smp_init_top(void *arg)
 		/* Initialize the dummy thread struct so that
 		 * the scheduler can schedule actual threads to run.
 		 */
-		z_dummy_thread_init(&_thread_dummies[arch_curr_cpu()->id]);
+		z_dummy_thread_init(&dummy_thread);
 	}
 
 #ifdef CONFIG_SYS_CLOCK_EXISTS