[PATCH v2] tracing: Ignore mmiotrace from kernel commandline

["Ziqian SUN (Zamir)" <zsun@redhat.com>]

From: "Ziqian SUN (Zamir)" <zsun@redhat.com>

The mmiotrace tracer cannot be enabled with ftrace=mmiotrace in
kernel commandline. With this patch, a trace_noboot_tracer_list
is implemented and will be checked during system boot. If the
tracer declares itself as not for boot up, system will print out
a message and continue booting.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=196557
Signed-off-by: Ziqian SUN (Zamir) <zsun@redhat.com>
---
 kernel/trace/trace.c           | 34 +++++++++++++++++++++++++++++++++-
 kernel/trace/trace.h           | 11 +++++++++++
 kernel/trace/trace_mmiotrace.c |  4 ++++
 3 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 44004d8..0d25b1e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -319,6 +319,33 @@ void trace_free_pid_list(struct trace_pid_list *pid_list)
 	kfree(pid_list);
 }
 
+struct mutex trace_noboot_tracer_lock;
+
+DEFINE_MUTEX(trace_noboot_tracer_lock);
+LIST_HEAD(trace_noboot_tracer_list);
+
+void trace_noboot_tracer_put(struct tracer_noboot *noboot_tracer)
+{
+	mutex_lock(&trace_noboot_tracer_lock);
+	list_add(&noboot_tracer->list, &trace_noboot_tracer_list);
+	mutex_unlock(&trace_noboot_tracer_lock);
+}
+
+bool trace_noboot_tracer_find(const char *str)
+{
+	mutex_lock(&trace_noboot_tracer_lock);
+	struct tracer_noboot *ptr = NULL;
+
+	list_for_each_entry(ptr, &trace_noboot_tracer_list, list) {
+		if (strcmp(ptr->name, str) == 0) {
+			mutex_unlock(&trace_noboot_tracer_lock);
+			return true;
+		}
+	}
+	mutex_unlock(&trace_noboot_tracer_lock);
+	return false;
+}
+
 /**
  * trace_find_filtered_pid - check if a pid exists in a filtered_pid list
  * @filtered_pids: The list of pids to check
@@ -1641,8 +1668,13 @@ int __init register_tracer(struct tracer *type)
 	if (strncmp(default_bootup_tracer, type->name, MAX_TRACER_SIZE))
 		goto out_unlock;
 
+	/* If the tracer should not be enabled by kernel parameter */
+	if (trace_noboot_tracer_find(type->name)) {
+		pr_warn("Tracer '%s' cannot be enabled during boot\n", type->name);
+		goto out_unlock;
+	}
+
 	printk(KERN_INFO "Starting tracer '%s'\n", type->name);
-	/* Do we want this tracer to start on bootup? */
 	tracing_set_tracer(&global_trace, type->name);
 	default_bootup_tracer = NULL;
 
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 490ba22..23bc668 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -502,6 +502,17 @@ enum {
 	TRACE_IRQ_BIT,
 };
 
+/*
+ * For tracer that cannot be enabled during boot time.
+ */
+
+struct tracer_noboot {
+	const char		*name;
+	struct list_head	list;
+};
+
+void trace_noboot_tracer_put(struct tracer_noboot *noboot_tracer);
+
 #define trace_recursion_set(bit)	do { (current)->trace_recursion |= (1<<(bit)); } while (0)
 #define trace_recursion_clear(bit)	do { (current)->trace_recursion &= ~(1<<(bit)); } while (0)
 #define trace_recursion_test(bit)	((current)->trace_recursion & (1<<(bit)))
diff --git a/kernel/trace/trace_mmiotrace.c b/kernel/trace/trace_mmiotrace.c
index cd7480d..06aa0a5 100644
--- a/kernel/trace/trace_mmiotrace.c
+++ b/kernel/trace/trace_mmiotrace.c
@@ -286,6 +286,10 @@ static enum print_line_t mmio_print_line(struct trace_iterator *iter)
 
 __init static int init_mmio_trace(void)
 {
+	struct tracer_noboot mmio_notrace;
+
+	mmio_notrace.name = "mmiotrace";
+	trace_noboot_tracer_put(&mmio_notrace);
 	return register_tracer(&mmio_tracer);
 }
 device_initcall(init_mmio_trace);
-- 
1.8.3.1