From 7d7f4e140c8bd720d0cc25f86ed902c5a13a17aa Mon Sep 17 00:00:00 2001
From: Gregory Nutt <gnutt@nuttx.org>
Date: Fri, 12 Feb 2016 08:15:16 -0600
Subject: [PATCH] OS: Add a RUNNABLE attribute to the tasklists to indicate if
 the task list includes the currently executing task.  Use this additional bit
 of information to determine if a context switch could really occur when
 removing a TCB from a task list

---
 arch                                 |  2 +-
 configs                              |  2 +-
 sched/init/os_start.c                | 12 ++++++++----
 sched/sched/sched.h                  |  2 ++
 sched/sched/sched_removereadytorun.c | 24 ++++++++++++++++++++++--
 5 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/arch b/arch
index 717113268b..a6ad88a85c 160000
--- a/arch
+++ b/arch
@@ -1 +1 @@
-Subproject commit 717113268b97f16d755a11609a63526dfe71e4cf
+Subproject commit a6ad88a85c9e4c6ab633fb53f53603539206a57c
diff --git a/configs b/configs
index 600d77e8b4..f9d09bf8c0 160000
--- a/configs
+++ b/configs
@@ -1 +1 @@
-Subproject commit 600d77e8b4631a67a6099b35a6959b6099b4a42b
+Subproject commit f9d09bf8c091cc2d8578ae572ad8f48bb93d062a
diff --git a/sched/init/os_start.c b/sched/init/os_start.c
index 4a5b776d93..d201553387 100644
--- a/sched/init/os_start.c
+++ b/sched/init/os_start.c
@@ -230,23 +230,27 @@ const struct tasklist_s g_tasklisttable[NUM_TASK_STATES] =
     &g_pendingtasks,
     TLIST_ATTR_PRIORITIZED
   },
+#ifdef CONFIG_SMP
   {                                              /* TSTATE_TASK_READYTORUN */
     &g_readytorun,
     TLIST_ATTR_PRIORITIZED
   },
-#ifdef CONFIG_SMP
   {                                              /* TSTATE_TASK_ASSIGNED */
     g_assignedtasks,
-    TLIST_ATTR_PRIORITIZED | TLIST_ATTR_INDEXED
+    TLIST_ATTR_PRIORITIZED | TLIST_ATTR_INDEXED | TLIST_ATTR_RUNNABLE
   },
   {                                              /* TSTATE_TASK_RUNNING */
     g_assignedtasks,
-    TLIST_ATTR_PRIORITIZED | TLIST_ATTR_INDEXED
+    TLIST_ATTR_PRIORITIZED | TLIST_ATTR_INDEXED | TLIST_ATTR_RUNNABLE
   },
 #else
+  {                                              /* TSTATE_TASK_READYTORUN */
+    &g_readytorun,
+    TLIST_ATTR_PRIORITIZED | TLIST_ATTR_RUNNABLE
+  },
   {                                              /* TSTATE_TASK_RUNNING */
     &g_readytorun,
-    TLIST_ATTR_PRIORITIZED
+    TLIST_ATTR_PRIORITIZED | TLIST_ATTR_RUNNABLE
   },
 #endif
   {                                              /* TSTATE_TASK_INACTIVE */
diff --git a/sched/sched/sched.h b/sched/sched/sched.h
index 2abdb59dfa..e48b421db6 100644
--- a/sched/sched/sched.h
+++ b/sched/sched/sched.h
@@ -82,10 +82,12 @@
 
 #define TLIST_ATTR_PRIORITIZED   (1 << 0) /* Bit 0: List is prioritized */
 #define TLIST_ATTR_INDEXED       (1 << 1) /* Bit 1: List is indexed by CPU */
+#define TLIST_ATTR_RUNNABLE      (1 << 2) /* Bit 2: List includes running tasks */
 
 #define __TLIST_ATTR(s)          g_tasklisttable[s].attr
 #define TLIST_ISPRIORITIZED(s)   ((__TLIST_ATTR(s) & TLIST_ATTR_PRIORITIZED) != 0)
 #define TLIST_ISINDEXED(s)       ((__TLIST_ATTR(s) & TLIST_ATTR_INDEXED) != 0)
+#define TLIST_ISRUNNABLE(s)      ((__TLIST_ATTR(s) & TLIST_ATTR_RUNNABLE) != 0)
 
 #define __TLIST_HEAD(s)          (FAR dq_queue_t *)g_tasklisttable[s].list
 #define __TLIST_HEADINDEXED(s,c) (&(__TLIST_HEAD(s))[c])
diff --git a/sched/sched/sched_removereadytorun.c b/sched/sched/sched_removereadytorun.c
index eb71e63892..4fa2407040 100644
--- a/sched/sched/sched_removereadytorun.c
+++ b/sched/sched/sched_removereadytorun.c
@@ -76,11 +76,31 @@ bool sched_removereadytorun(FAR struct tcb_s *rtcb)
   FAR dq_queue_t *tasklist;
   bool ret = false;
 
-  /* Check if the TCB to be removed is at the head of the ready to run list.
-   * In this case, we are removing the currently active task on this CPU.
+  /* Check if the TCB to be removed is at the head of a ready to run list. */
+
+#ifdef CONFIG_SMP
+  /* For the case of SMP, there are two lists involved:  (1) the
+   * g_readytorun list that holds non-running tasks that have not been
+   * assigned to a CPU, and (2) and the g_assignedtasks[] lists which hold
+   * tasks assigned a CPU, including the task that is currently running on
+   * that CPU.  Only this latter list contains the currently active task
+   * only only removing the head of that list can result in a context
+   * switch.
+   *
+   * The tasklist RUNNABLE attribute will inform us if the list holds the
+   * currently executing and task and, hence, if a context switch could
+   * occur.
+   */
+
+  if (!rtcb->blink || TLIST_ISRUNNABLE(rtcb->task_state))
+#else
+  /* There is only one list, g_readytorun, and it always contains the
+   * currently running task.  If we are removing the head of this list,
+   * then we are removing the currently active task.
    */
 
   if (!rtcb->blink)
+#endif
     {
       /* There must always be at least one task in the list (the idle task) */