Add EXPERIMENTAL configuration option to investigate some SMP issues

include/nuttx/spinlock.h

@@ -63,10 +63,14 @@
 
 struct spinlock_s
 {
-  spinlock_t sp_lock;  /* Indicates if the spinlock is locked or not.  See the
-                        * values SP_LOCKED and SP_UNLOCKED. */
-  uint8_t    sp_cpu;   /* CPU holding the lock */
-  uint16_t   sp_count; /* The count of references by this CPU on the lock */
+  volatile spinlock_t sp_lock;  /* Indicates if the spinlock is locked or
+                                 * not.  See the* values SP_LOCKED and
+                                 * SP_UNLOCKED. */
+#ifdef CONFIG_SMP
+  uint8_t  sp_cpu;              /* CPU holding the lock */
+  uint16_t sp_count;            /* The count of references by this CPU on
+                                 * the lock */
+#endif
 };
 
 /****************************************************************************
@@ -93,7 +97,7 @@ struct spinlock_s
  *
  ****************************************************************************/
 
-spinlock_t up_testset(FAR spinlock_t *lock);
+spinlock_t up_testset(volatile FAR spinlock_t *lock);
 
 /****************************************************************************
  * Name: spinlock_initialize

sched/Kconfig

@@ -229,6 +229,35 @@ endmenu # Clocks and Timers
 
 menu "Tasks and Scheduling"
 
+config SPINLOCK
+	bool "Support Spinlocks"
+	default n
+	depends on EXPERIMENTAL
+	---help---
+		Enables suppport for spinlocks.  Spinlocks are current used only for
+		SMP suppport.
+
+config SMP
+	bool "Symmetric Multi-Processing (SMP)"
+	default n
+	depends on EXPERIMENTAL
+	select SPINLOCK
+	---help---
+		Enable support for Symmetric Multi-Processing (SMP) on a multi-CPU
+		platform.
+
+if SMP
+
+config SMP_NCPUS
+	int "Number of CPUs"
+	default 4
+	range 2 32
+	---help---
+		This value identifies the number of CPUs support by the processor
+		that will be used for SMP.
+
+endif # SMP
+
 choice
 	prompt "Initialization Task"
 	default INIT_ENTRYPOINT if !BUILD_KERNEL
@@ -307,11 +336,6 @@ config RR_INTERVAL
 		Round roben scheduling (SCHED_RR) is enabled by setting this
 		interval to a positive, non-zero value.
 
-config SPINLOCK
-	bool "Support Spinlocks"
-	default n
-	depends on EXPERIMENTAL
-
 config SCHED_SPORADIC
 	bool "Support sporadic scheduling"
 	default n

sched/semaphore/spinlock.c

@@ -86,8 +86,10 @@ void spinlock_initialize(FAR struct spinlock_s *lock)
   DEBUGASSERT(lock != NULL);
 
   lock->sp_lock  = SP_UNLOCKED;
+#ifdef CONFIG_SMP
   lock->sp_cpu   = IMPOSSIBLE_CPU;
   lock->sp_count = 0;
+#endif
 }
 
 /****************************************************************************
@@ -111,6 +113,7 @@ void spinlock_initialize(FAR struct spinlock_s *lock)
 
 void spinlock(FAR struct spinlock_s *lock)
 {
+#ifdef CONFIG_SMP
   irqstate_t flags;
   uint8_t cpu = this_cpu();
 
@@ -158,6 +161,20 @@ void spinlock(FAR struct spinlock_s *lock)
     }
 
   irqrestore(flags);
+
+#else /* CONFIG_SMP */
+
+  /* Take the lock.  REVISIT:  We should set an indication in the TCB that
+   * the thread is spinning.  This might be useful in determining some
+   * scheduling actions?
+   */
+
+  while (up_testset(&lock->sp_lock) == SP_LOCKED)
+    {
+      sched_yield();
+    }
+
+#endif /* CONFIG_SMP */
 }
 
 /****************************************************************************
@@ -179,6 +196,7 @@ void spinlock(FAR struct spinlock_s *lock)
 
 void spinunlock(FAR struct spinlock_s *lock)
 {
+#ifdef CONFIG_SMP
   irqstate_t flags;
 #ifdef CONFIG_SPINLOCK_LOCKDOWN
   uint8_t cpu = this_cpu();
@@ -225,6 +243,14 @@ void spinunlock(FAR struct spinlock_s *lock)
     }
 
   irqrestore(flags);
+
+#else /* CONFIG_SMP */
+  /* Just mark the spinlock unlocked */
+
+  DEBUGASSERT(lock != NULL && lock->sp-lock = SP_LOCKED);
+  lock->sp_lock  = SP_UNLOCKED; 
+
+#endif /* CONFIG_SMP */
 }
 
 #endif /* CONFIG_SPINLOCK */