cgroup/tracing: Move taking of spin lock out of trace event handlers

It is unwise to take spin locks from the handlers of trace events.
Mainly, because they can introduce lockups, because it introduces locks
in places that are normally not tested. Worse yet, because trace events
are tucked away in the include/trace/events/ directory, locks that are
taken there are forgotten about.

As a general rule, I tell people never to take any locks in a trace
event handler.

Several cgroup trace event handlers call cgroup_path() which eventually
takes the kernfs_rename_lock spinlock. This injects the spinlock in the
code without people realizing it. It also can cause issues for the
PREEMPT_RT patch, as the spinlock becomes a mutex, and the trace event
handlers are called with preemption disabled.

By moving the calculation of the cgroup_path() out of the trace event
handlers and into a macro (surrounded by a
trace_cgroup_##type##_enabled()), then we could place the cgroup_path
into a string, and pass that to the trace event. Not only does this
remove the taking of the spinlock out of the trace event handler, but
it also means that the cgroup_path() only needs to be called once (it
is currently called twice, once to get the length to reserver the
buffer for, and once again to get the path itself. Now it only needs to
be done once.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/trace/events/cgroup.h

@@ -53,24 +53,22 @@ DEFINE_EVENT(cgroup_root, cgroup_remount,
 
 DECLARE_EVENT_CLASS(cgroup,
 
-	TP_PROTO(struct cgroup *cgrp),
+	TP_PROTO(struct cgroup *cgrp, const char *path),
 
-	TP_ARGS(cgrp),
+	TP_ARGS(cgrp, path),
 
 	TP_STRUCT__entry(
 		__field(	int,		root			)
 		__field(	int,		id			)
 		__field(	int,		level			)
-		__dynamic_array(char,		path,
-				cgroup_path(cgrp, NULL, 0) + 1)
+		__string(	path,		path			)
 	),
 
 	TP_fast_assign(
 		__entry->root = cgrp->root->hierarchy_id;
 		__entry->id = cgrp->id;
 		__entry->level = cgrp->level;
-		cgroup_path(cgrp, __get_dynamic_array(path),
-				  __get_dynamic_array_len(path));
+		__assign_str(path, path);
 	),
 
 	TP_printk("root=%d id=%d level=%d path=%s",
@@ -79,54 +77,53 @@ DECLARE_EVENT_CLASS(cgroup,
 
 DEFINE_EVENT(cgroup, cgroup_mkdir,
 
-	TP_PROTO(struct cgroup *cgroup),
+	TP_PROTO(struct cgroup *cgrp, const char *path),
 
-	TP_ARGS(cgroup)
+	TP_ARGS(cgrp, path)
 );
 
 DEFINE_EVENT(cgroup, cgroup_rmdir,
 
-	TP_PROTO(struct cgroup *cgroup),
+	TP_PROTO(struct cgroup *cgrp, const char *path),
 
-	TP_ARGS(cgroup)
+	TP_ARGS(cgrp, path)
 );
 
 DEFINE_EVENT(cgroup, cgroup_release,
 
-	TP_PROTO(struct cgroup *cgroup),
+	TP_PROTO(struct cgroup *cgrp, const char *path),
 
-	TP_ARGS(cgroup)
+	TP_ARGS(cgrp, path)
 );
 
 DEFINE_EVENT(cgroup, cgroup_rename,
 
-	TP_PROTO(struct cgroup *cgroup),
+	TP_PROTO(struct cgroup *cgrp, const char *path),
 
-	TP_ARGS(cgroup)
+	TP_ARGS(cgrp, path)
 );
 
 DECLARE_EVENT_CLASS(cgroup_migrate,
 
-	TP_PROTO(struct cgroup *dst_cgrp, struct task_struct *task, bool threadgroup),
+	TP_PROTO(struct cgroup *dst_cgrp, const char *path,
+		 struct task_struct *task, bool threadgroup),
 
-	TP_ARGS(dst_cgrp, task, threadgroup),
+	TP_ARGS(dst_cgrp, path, task, threadgroup),
 
 	TP_STRUCT__entry(
 		__field(	int,		dst_root		)
 		__field(	int,		dst_id			)
 		__field(	int,		dst_level		)
-		__dynamic_array(char,		dst_path,
-				cgroup_path(dst_cgrp, NULL, 0) + 1)
 		__field(	int,		pid			)
+		__string(	dst_path,	path			)
 		__string(	comm,		task->comm		)
 	),
 
 	TP_fast_assign(
 		__entry->dst_root = dst_cgrp->root->hierarchy_id;
 		__entry->dst_id = dst_cgrp->id;
 		__entry->dst_level = dst_cgrp->level;
-		cgroup_path(dst_cgrp, __get_dynamic_array(dst_path),
-				      __get_dynamic_array_len(dst_path));
+		__assign_str(dst_path, path);
 		__entry->pid = task->pid;
 		__assign_str(comm, task->comm);
 	),
@@ -138,16 +135,18 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
 
 DEFINE_EVENT(cgroup_migrate, cgroup_attach_task,
 
-	TP_PROTO(struct cgroup *dst_cgrp, struct task_struct *task, bool threadgroup),
+	TP_PROTO(struct cgroup *dst_cgrp, const char *path,
+		 struct task_struct *task, bool threadgroup),
 
-	TP_ARGS(dst_cgrp, task, threadgroup)
+	TP_ARGS(dst_cgrp, path, task, threadgroup)
 );
 
 DEFINE_EVENT(cgroup_migrate, cgroup_transfer_tasks,
 
-	TP_PROTO(struct cgroup *dst_cgrp, struct task_struct *task, bool threadgroup),
+	TP_PROTO(struct cgroup *dst_cgrp, const char *path,
+		 struct task_struct *task, bool threadgroup),
 
-	TP_ARGS(dst_cgrp, task, threadgroup)
+	TP_ARGS(dst_cgrp, path, task, threadgroup)
 );
 
 #endif /* _TRACE_CGROUP_H */

kernel/cgroup/cgroup-internal.h

@@ -8,6 +8,32 @@
 #include <linux/list.h>
 #include <linux/refcount.h>
 
+#define TRACE_CGROUP_PATH_LEN 1024
+extern spinlock_t trace_cgroup_path_lock;
+extern char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
+
+/*
+ * cgroup_path() takes a spin lock. It is good practice not to take
+ * spin locks within trace point handlers, as they are mostly hidden
+ * from normal view. As cgroup_path() can take the kernfs_rename_lock
+ * spin lock, it is best to not call that function from the trace event
+ * handler.
+ *
+ * Note: trace_cgroup_##type##_enabled() is a static branch that will only
+ *       be set when the trace event is enabled.
+ */
+#define TRACE_CGROUP_PATH(type, cgrp, ...)				\
+	do {								\
+		if (trace_cgroup_##type##_enabled()) {			\
+			spin_lock(&trace_cgroup_path_lock);		\
+			cgroup_path(cgrp, trace_cgroup_path,		\
+				    TRACE_CGROUP_PATH_LEN);		\
+			trace_cgroup_##type(cgrp, trace_cgroup_path,	\
+					    ##__VA_ARGS__);		\
+			spin_unlock(&trace_cgroup_path_lock);		\
+		}							\
+	} while (0)
+
 /*
  * A cgroup can be associated with multiple css_sets as different tasks may
  * belong to different cgroups on different hierarchies.  In the other

kernel/cgroup/cgroup-v1.c

@@ -135,7 +135,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
 		if (task) {
 			ret = cgroup_migrate(task, false, &mgctx);
 			if (!ret)
-				trace_cgroup_transfer_tasks(to, task, false);
+				TRACE_CGROUP_PATH(transfer_tasks, to, task, false);
 			put_task_struct(task);
 		}
 	} while (task && !ret);
@@ -865,7 +865,7 @@ static int cgroup1_rename(struct kernfs_node *kn, struct kernfs_node *new_parent
 
 	ret = kernfs_rename(kn, new_parent, new_name_str);
 	if (!ret)
-		trace_cgroup_rename(cgrp);
+		TRACE_CGROUP_PATH(rename, cgrp);
 
 	mutex_unlock(&cgroup_mutex);
 

kernel/cgroup/cgroup.c

@@ -83,6 +83,9 @@ EXPORT_SYMBOL_GPL(cgroup_mutex);
 EXPORT_SYMBOL_GPL(css_set_lock);
 #endif
 
+DEFINE_SPINLOCK(trace_cgroup_path_lock);
+char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
+
 /*
  * Protects cgroup_idr and css_idr so that IDs can be released without
  * grabbing cgroup_mutex.
@@ -2638,7 +2641,7 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 	cgroup_migrate_finish(&mgctx);
 
 	if (!ret)
-		trace_cgroup_attach_task(dst_cgrp, leader, threadgroup);
+		TRACE_CGROUP_PATH(attach_task, dst_cgrp, leader, threadgroup);
 
 	return ret;
 }
@@ -4634,7 +4637,7 @@ static void css_release_work_fn(struct work_struct *work)
 		struct cgroup *tcgrp;
 
 		/* cgroup release path */
-		trace_cgroup_release(cgrp);
+		TRACE_CGROUP_PATH(release, cgrp);
 
 		if (cgroup_on_dfl(cgrp))
 			cgroup_rstat_flush(cgrp);
@@ -4977,7 +4980,7 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 	if (ret)
 		goto out_destroy;
 
-	trace_cgroup_mkdir(cgrp);
+	TRACE_CGROUP_PATH(mkdir, cgrp);
 
 	/* let's create and online css's */
 	kernfs_activate(kn);
@@ -5165,9 +5168,8 @@ int cgroup_rmdir(struct kernfs_node *kn)
 		return 0;
 
 	ret = cgroup_destroy_locked(cgrp);
-
 	if (!ret)
-		trace_cgroup_rmdir(cgrp);
+		TRACE_CGROUP_PATH(rmdir, cgrp);
 
 	cgroup_kn_unlock(kn);
 	return ret;