exec: Remove cpu from cpus list during cpu_exec_exit()

CPUState *cpu gets added to the cpus list during cpu_exec_init(). It
should be removed from cpu_exec_exit().

cpu_exec_exit() is called from generic CPU::instance_finalize and some
archs like PowerPC call it from CPU unrealizefn. So ensure that we
dequeue the cpu only once.

Now -1 value for cpu->cpu_index indicates that we have already dequeued
the cpu for CONFIG_USER_ONLY case also.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

exec.c

@@ -612,15 +612,9 @@ static int cpu_get_free_index(Error **errp)
     return cpu;
 }
 
-void cpu_exec_exit(CPUState *cpu)
+static void cpu_release_index(CPUState *cpu)
 {
-    if (cpu->cpu_index == -1) {
-        /* cpu_index was never allocated by this @cpu or was already freed. */
-        return;
-    }
-
     bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
-    cpu->cpu_index = -1;
 }
 #else
 
@@ -635,11 +629,33 @@ static int cpu_get_free_index(Error **errp)
     return cpu_index;
 }
 
-void cpu_exec_exit(CPUState *cpu)
+static void cpu_release_index(CPUState *cpu)
 {
+    return;
 }
 #endif
 
+void cpu_exec_exit(CPUState *cpu)
+{
+#if defined(CONFIG_USER_ONLY)
+    cpu_list_lock();
+#endif
+    if (cpu->cpu_index == -1) {
+        /* cpu_index was never allocated by this @cpu or was already freed. */
+#if defined(CONFIG_USER_ONLY)
+        cpu_list_unlock();
+#endif
+        return;
+    }
+
+    QTAILQ_REMOVE(&cpus, cpu, node);
+    cpu_release_index(cpu);
+    cpu->cpu_index = -1;
+#if defined(CONFIG_USER_ONLY)
+    cpu_list_unlock();
+#endif
+}
+
 void cpu_exec_init(CPUState *cpu, Error **errp)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);