In order to stop branch tracing for a running task, we need to first
clear the branch tracing control bits before we may free the tracing
buffer.
If the traced task is running, the cpu might still trace that task
after the branch trace control bits have cleared.

Wait until the traced task has been scheduled out before proceeding.


A similar problem affects the task debug store context. We first remove
the context, then we need to wait until the task has been scheduled
out before we can free the context memory.


Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
---
 arch/x86/kernel/ds.c |   40 	40 +	0 -	0 !
 1 file changed, 40 insertions(+)

Index: b/arch/x86/kernel/ds.c
===================================================================
--- a/arch/x86/kernel/ds.c
+++ b/arch/x86/kernel/ds.c
@@ -250,6 +250,40 @@ static DEFINE_PER_CPU(struct ds_context 
 #define system_context per_cpu(system_context_array, smp_processor_id())
 
 
+/*
+ * Wait for the traced task to unschedule.
+ *
+ * This guarantees that the bts trace configuration has been
+ * synchronized with the cpu executing the task.
+ */
+static void wait_to_unschedule(struct task_struct *task)
+{
+	unsigned long nvcsw;
+	unsigned long nivcsw;
+
+	if (!task)
+		return;
+
+	if (task == current)
+		return;
+
+	nvcsw  = task->nvcsw;
+	nivcsw = task->nivcsw;
+	for (;;) {
+		if (!task_is_running(task))
+			break;
+		/*
+		 * The switch count is incremented before the actual
+		 * context switch. We thus wait for two switches to be
+		 * sure at least one completed.
+		 */
+		if ((task->nvcsw - nvcsw) > 1)
+			break;
+		if ((task->nivcsw - nivcsw) > 1)
+			break;
+	}
+}
+
 static inline struct ds_context *ds_get_context(struct task_struct *task)
 {
 	struct ds_context **p_context =
@@ -321,6 +355,9 @@ static inline void ds_put_context(struct
 
 	spin_unlock_irqrestore(&ds_lock, irq);
 
+	/* The context might still be in use for context switching. */
+	wait_to_unschedule(context->task);
+
 	kfree(context);
 }
 
@@ -789,6 +826,9 @@ void ds_release_bts(struct bts_tracer *t
 	WARN_ON_ONCE(tracer->ds.context->bts_master != tracer);
 	tracer->ds.context->bts_master = NULL;
 
+	/* Make sure tracing stopped and the tracer is not in use. */
+	wait_to_unschedule(tracer->ds.context->task);
+
 	put_tracer(tracer->ds.context->task);
 	ds_put_context(tracer->ds.context);
 

--