[arch][arm] add debug asserts to the non SMP spinlock routines

This should assert that the spinlock is not already held when acquiring
and vice-versa and that interrupts are disabled.

app/tests/thread_tests.c

@@ -558,12 +558,14 @@ static void spinlock_test(void) {
     printf("seems to work\n");
 
 #define COUNT (1024*1024)
+    arch_interrupt_save(&state, SPIN_LOCK_FLAG_INTERRUPTS);
     uint32_t c = arch_cycle_count();
     for (uint i = 0; i < COUNT; i++) {
         spin_lock(&lock);
         spin_unlock(&lock);
     }
     c = arch_cycle_count() - c;
+    arch_interrupt_restore(state, SPIN_LOCK_FLAG_INTERRUPTS);
 
     printf("%u cycles to acquire/release lock %u times (%u cycles per)\n", c, COUNT, c / COUNT);
 

arch/arm/include/arch/spinlock.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <lk/compiler.h>
+#include <assert.h>
 #include <arch/ops.h>
 #include <stdbool.h>
 
@@ -36,15 +37,22 @@ void arch_spin_unlock(spin_lock_t *lock);
 
 #else
 
+/* Non-SMP spinlocks are mostly vestigial to try to catch pending locking problems. */
 static inline void arch_spin_lock(spin_lock_t *lock) {
+    DEBUG_ASSERT(arch_ints_disabled());
+    DEBUG_ASSERT(*lock == 0);
     *lock = 1;
 }
 
 static inline int arch_spin_trylock(spin_lock_t *lock) {
+    DEBUG_ASSERT(arch_ints_disabled());
+    DEBUG_ASSERT(*lock == 0);
     return 0;
 }
 
 static inline void arch_spin_unlock(spin_lock_t *lock) {
+    DEBUG_ASSERT(arch_ints_disabled());
+    DEBUG_ASSERT(*lock != 0);
     *lock = 0;
 }