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>

1 files changed, 7 insertions, 5 deletions

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 077370bf8964..4a439de621bd 100644
--- a/kernel/cgroup/cgroup.c
+++ b/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;