SMP: Enforce this rule: Tasks which are normally restored when sched_unlock() is called must remain pending (1) if we are in a critical section, i.e., g_cpu_irqlock is locked , or (2) other CPUs still have pre-emption disabled, i.e., g_cpu_schedlock is locked. In those cases, the release of the pending tasks must be deferred until those conditions are met.

ChangeLog

@@ -13466,5 +13466,13 @@
 	  (2016-12-24).
 	* STM32 F4: Allow dma in 1 bit mode in STM32F4xxx.  From David Sidrane
 	  (2016-12-24).
+	* termios.h:  Fix CRTSCTS define to include input and output flow.
+	  From  Lorenz Meier (2016-12-26).
+	* SMP: Enforce this rule: Tasks which are normally restored when
+	  sched_unlock() is called must remain pending (1) if we are in a
+	  critical section, i.e., g_cpu_irqlock is locked , or (2) other CPUs
+	  still have pre-emption disabled, i.e., g_cpu_schedlock is locked.  In
+	  those cases, the release of the pending tasks must be deferred until
+	  those conditions are met (2016-12-26).
 
 7.20 2017-xx-xx Gregory Nutt <gnutt@nuttx.org>

sched/irq/irq_csection.c

@@ -513,12 +513,9 @@ void leave_critical_section(irqstate_t flags)
               if (!spin_islocked(&g_cpu_irqlock))
                 {
                   /* Check if there are pending tasks and that pre-emption
-                   * is also enabled.
+                   * is also enabled.  This is necessary becaue we may have
-                   *
+                   * deferred the up_release_pending() call in sched_unlock()
-                   * REVISIT: Is there an issue here?  up_release_pending()
+                   * because we were within a critical section then.
-                   * must be called from within a critical section but here
-                   * we have just left the critical section.  At least we
-                   * still have interrupts disabled on this CPU.
                    */
 
                   if (g_pendingtasks.head != NULL &&
@@ -529,9 +526,6 @@ void leave_critical_section(irqstate_t flags)
                        *
                        * NOTE: This operation has a very high likelihood of
                        * causing this task to be switched out!
-                       *
-                       * REVISIT: Should this not be done while we are in the
-                       * critical section.
                        */
 
                       up_release_pending();

sched/sched/sched_unlock.c

@@ -44,6 +44,7 @@
 #include <nuttx/arch.h>
 #include <nuttx/sched_note.h>
 
+#include "irq/irq.h"
 #include "sched/sched.h"
 
 /****************************************************************************
@@ -113,16 +114,30 @@ int sched_unlock(void)
 #endif
 
           /* Release any ready-to-run tasks that have collected in
-           * g_pendingtasks.  In the SMP case, the scheduler remains
+           * g_pendingtasks.
-           * locked if interrupts are disabled.
            *
            * NOTE: This operation has a very high likelihood of causing
            * this task to be switched out!
            */
 
 #ifdef CONFIG_SMP
-          if (!spin_islocked(&g_cpu_schedlock) && g_pendingtasks.head != NULL)
+          /* In the SMP case, the tasks remains pend(1) if we are
+           * in a critical section, i.e., g_cpu_irqlock is locked , or (2)
+           * other CPUs still have pre-emption disabled, i.e.,
+           * g_cpu_schedlock is locked.  In those cases, the release of the
+           * pending tasks must be deferred until those conditions are met.ing 
+           */
+
+          if (!spin_islocked(&g_cpu_schedlock) &&
+              !spin_islocked(&g_cpu_irqlock) &&
+              g_pendingtasks.head != NULL)
 #else
+          /* In the single CPU case, decrementing irqcount to zero is
+           * sufficient to release the pending tasks.  Further, in that
+           * configuration, critical sections and pre-emption can operate
+           * fully independently.
+           */
+
           if (g_pendingtasks.head != NULL)
 #endif
             {