cgroup: Restructure release_agent_path handling

cgrp->root->release_agent_path is protected by both cgroup_mutex and
release_agent_path_lock and readers can hold either one. The
dual-locking scheme was introduced while breaking a locking dependency
issue around cgroup_mutex but doesn't make sense anymore given that
the only remaining reader which uses cgroup_mutex is
cgroup1_releaes_agent().

This patch updates cgroup1_release_agent() to use
release_agent_path_lock so that release_agent_path is always protected
only by release_agent_path_lock.

While at it, convert strlen() based empty string checks to direct
tests on the first character as suggested by Linus.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup/cgroup-v1.c

@@ -38,10 +38,7 @@ static bool cgroup_no_v1_named;
  */
 static struct workqueue_struct *cgroup_pidlist_destroy_wq;
 
-/*
- * Protects cgroup_subsys->release_agent_path.  Modifying it also requires
- * cgroup_mutex.  Reading requires either cgroup_mutex or this spinlock.
- */
+/* protects cgroup_subsys->release_agent_path */
 static DEFINE_SPINLOCK(release_agent_path_lock);
 
 bool cgroup1_ssid_disabled(int ssid)
@@ -775,22 +772,29 @@ void cgroup1_release_agent(struct work_struct *work)
 {
 	struct cgroup *cgrp =
 		container_of(work, struct cgroup, release_agent_work);
-	char *pathbuf = NULL, *agentbuf = NULL;
+	char *pathbuf, *agentbuf;
 	char *argv[3], *envp[3];
 	int ret;
 
-	mutex_lock(&cgroup_mutex);
+	/* snoop agent path and exit early if empty */
+	if (!cgrp->root->release_agent_path[0])
+		return;
 
+	/* prepare argument buffers */
 	pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
-	agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
-	if (!pathbuf || !agentbuf || !strlen(agentbuf))
-		goto out;
+	agentbuf = kmalloc(PATH_MAX, GFP_KERNEL);
+	if (!pathbuf || !agentbuf)
+		goto out_free;
 
-	spin_lock_irq(&css_set_lock);
-	ret = cgroup_path_ns_locked(cgrp, pathbuf, PATH_MAX, &init_cgroup_ns);
-	spin_unlock_irq(&css_set_lock);
+	spin_lock(&release_agent_path_lock);
+	strlcpy(agentbuf, cgrp->root->release_agent_path, PATH_MAX);
+	spin_unlock(&release_agent_path_lock);
+	if (!agentbuf[0])
+		goto out_free;
+
+	ret = cgroup_path_ns(cgrp, pathbuf, PATH_MAX, &init_cgroup_ns);
 	if (ret < 0 || ret >= PATH_MAX)
-		goto out;
+		goto out_free;
 
 	argv[0] = agentbuf;
 	argv[1] = pathbuf;
@@ -801,11 +805,7 @@ void cgroup1_release_agent(struct work_struct *work)
 	envp[1] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
 	envp[2] = NULL;
 
-	mutex_unlock(&cgroup_mutex);
 	call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
-	goto out_free;
-out:
-	mutex_unlock(&cgroup_mutex);
 out_free:
 	kfree(agentbuf);
 	kfree(pathbuf);