Synchronize plugin switching

Make sure the plugin is not used by any CPUs while switching.
The CPU performing the switch sends an IPI to all other CPUs forcing
them to synchronize on an atomic variable.

1 files changed, 15 insertions, 3 deletions

diff --git a/litmus/litmus.c b/litmus/litmus.c
index 3cf7cb9e8a9f..3ef2df8ffb50 100644
--- a/litmus/litmus.c
+++ b/litmus/litmus.c
@@ -24,6 +24,8 @@
 /* Number of RT tasks that exist in the system */
 atomic_t rt_task_count          = ATOMIC_INIT(0);
 static DEFINE_SPINLOCK(task_transition_lock);
+/* synchronize plugin switching */
+atomic_t cannot_use_plugin      = ATOMIC_INIT(0);
 
 /* Give log messages sequential IDs. */
 atomic_t __log_seq_no = ATOMIC_INIT(0);
@@ -369,13 +371,17 @@ void litmus_exit_task(struct task_struct* tsk)
         }
 }
 
+/* IPI callback to synchronize plugin switching */
+static void synch_on_plugin_switch(void* info)
+{
+        while (atomic_read(&cannot_use_plugin))
+                cpu_relax();
+}
+
 /* Switching a plugin in use is tricky.
  * We must watch out that no real-time tasks exists
  * (and that none is created in parallel) and that the plugin is not
  * currently in use on any processor (in theory).
- *
- * For now, we don't enforce the second part since it is unlikely to cause
- * any trouble by itself as long as we don't unload modules.
  */
 int switch_sched_plugin(struct sched_plugin* plugin)
 {
@@ -384,6 +390,11 @@ int switch_sched_plugin(struct sched_plugin* plugin)
 
         BUG_ON(!plugin);
 
+        /* forbid other cpus to use the plugin */
+        atomic_set(&cannot_use_plugin, 1);
+        /* send IPI to force other CPUs to synch with us */
+        smp_call_function(synch_on_plugin_switch, NULL, 0);
+
         /* stop task transitions */
         spin_lock_irqsave(&task_transition_lock, flags);
 
@@ -404,6 +415,7 @@ int switch_sched_plugin(struct sched_plugin* plugin)
                 ret = -EBUSY;
 out:
         spin_unlock_irqrestore(&task_transition_lock, flags);
+        atomic_set(&cannot_use_plugin, 0);
         return ret;
 }