sched/sched: Clean up some logic that I committed yesterday. Add more comments and conditional logic to clarify the issues.

sched/sched/Make.defs

@@ -100,8 +100,10 @@ endif
 
 ifeq ($(CONFIG_SMP),y)
 CSRCS += sched_tasklistlock.c
+ifeq ($(CONFIG_ARCH_GLOBAL_IRQDISABLE),y)
 CSRCS += sched_thistask.c
 endif
+endif
 
 # Include sched build support
 

sched/sched/sched.h

@@ -72,12 +72,19 @@
 
 /* These are macros to access the current CPU and the current task on a CPU.
  * These macros are intended to support a future SMP implementation.
- * NOTE: this_task() for SMP is implemented in sched_thistask.c
+ * NOTE: this_task() for SMP is implemented in sched_thistask.c if the CPU
+ * supports disabling of inter-processor interrupts.
+ *
+ * REVISIT: A mechanism to provide an atomic this_task() is still required
+ * for the case where where inter-processor interrupts cannot be disabled!
  */
 
 #ifdef CONFIG_SMP
 #  define current_task(cpu)      ((FAR struct tcb_s *)g_assignedtasks[cpu].head)
 #  define this_cpu()             up_cpu_index()
+#  ifndef CONFIG_ARCH_GLOBAL_IRQDISABLE
+#    define this_task()          (current_task(this_cpu()))
+#  endif
 #else
 #  define current_task(cpu)      ((FAR struct tcb_s *)g_readytorun.head)
 #  define this_cpu()             (0)
@@ -428,7 +435,9 @@ void sched_sporadic_lowpriority(FAR struct tcb_s *tcb);
 #endif
 
 #ifdef CONFIG_SMP
+#ifdef CONFIG_ARCH_GLOBAL_IRQDISABLE
 FAR struct tcb_s *this_task(void);
+#endif
 
 int  sched_cpu_select(cpu_set_t affinity);
 int  sched_cpu_pause(FAR struct tcb_s *tcb);

sched/sched/sched_thistask.c

@@ -69,17 +69,22 @@
 
 FAR struct tcb_s *this_task(void)
 {
-  irqstate_t flags;
   FAR struct tcb_s *tcb;
-
 #ifdef CONFIG_ARCH_GLOBAL_IRQDISABLE
-  /* Disable local interrupts to avoid CPU switching */
+  irqstate_t flags;
+
+  /* If the CPU supports suppression of interprocessor interrupts, then simple
+   * disabling interrupts will provide sufficient protection for the following
+   * operations.
+   */
 
   flags = up_irq_save();
 #else
-  /* Enter a critical section */
+  /* REVISIT:  Otherwise, there is no protection available.  sched_lock() and
-
+   * enter_critical section are not viable options here (because both depend
-  //flags = enter_critical_section();
+   * on this_task()).
+   */
+#  warning "Missing critical section"
 #endif
 
   /* Obtain the TCB which is currently running on this CPU */
@@ -90,8 +95,6 @@ FAR struct tcb_s *this_task(void)
 
 #ifdef CONFIG_ARCH_GLOBAL_IRQDISABLE
   up_irq_restore(flags);
-#else
-  //leave_critical_section(flags);
 #endif
   return tcb;
 }