Xtensa SMP: Avoid a nasty situation in SMP by assuring that up_release_pending() is not re-entered.

arch/xtensa/src/common/xtensa_cpupause.c

@@ -229,6 +229,9 @@ int up_cpu_pause(int cpu)
    * handler from returning until up_cpu_resume() is called; g_cpu_paused
    * is a handshake that will prefent this function from returning until
    * the CPU is actually paused.
+   *
+   * REVISIT: OR should there be logic to just skip this if the other CPU
+   * is already paused.
    */
 
   DEBUGASSERT(!spin_islocked(&g_cpu_wait[cpu]) &&
@@ -297,6 +300,9 @@ int up_cpu_resume(int cpu)
   /* Release the spinlock.  Releasing the spinlock will cause the SGI2
    * handler on 'cpu' to continue and return from interrupt to the newly
    * established thread.
+   *
+   * REVISIT: Should there be a more positive handshake to assure that the
+   * resumption is complete before returning.
    */
 
   DEBUGASSERT(spin_islocked(&g_cpu_wait[cpu]) &&

arch/xtensa/src/common/xtensa_releasepending.c

@@ -39,6 +39,7 @@
 
 #include <nuttx/config.h>
 
+#include <stdbool.h>
 #include <sched.h>
 #include <debug.h>
 
@@ -68,9 +69,24 @@
 void up_release_pending(void)
 {
   struct tcb_s *rtcb = this_task();
+#ifdef CONFIG_SMP
+  static bool busy = false;
+#endif
 
   sinfo("From TCB=%p\n", rtcb);
 
+  /* In SMP configurations, this function will be called as part of leaving
+   * the critical section.  In that case, it may be re-entered as part of
+   * the sched_addreadytorun() processing.  We have to guard against that
+   * case.
+   */
+
+#ifdef CONFIG_SMP
+  if (!busy)
+    {
+      busy = true;
+#endif
+
       /* Merge the g_pendingtasks list into the ready-to-run task list */
 
       /* sched_lock(); */
@@ -156,4 +172,9 @@ void up_release_pending(void)
               xtensa_context_restore(rtcb->xcp.regs);
             }
         }
+
+#ifdef CONFIG_SMP
+      busy = false;
+    }
+#endif
 }