workqueue: implement current_is_async()

This function queries whether %current is an async worker executing an
async item.  This will be used to implement warning on synchronous
request_module() from async workers.
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/async.h

@@ -52,4 +52,5 @@ extern void async_synchronize_full_domain(struct async_domain *domain);
 extern void async_synchronize_cookie(async_cookie_t cookie);
 extern void async_synchronize_cookie_domain(async_cookie_t cookie,
 					    struct async_domain *domain);
+extern bool current_is_async(void);
 #endif

kernel/async.c

@@ -57,6 +57,8 @@ asynchronous and synchronous parts of the kernel.
 #include <linux/slab.h>
 #include <linux/workqueue.h>
 
+#include "workqueue_internal.h"
+
 static async_cookie_t next_cookie = 1;
 
 #define MAX_WORK	32768
@@ -337,3 +339,15 @@ void async_synchronize_cookie(async_cookie_t cookie)
 	async_synchronize_cookie_domain(cookie, &async_running);
 }
 EXPORT_SYMBOL_GPL(async_synchronize_cookie);
+
+/**
+ * current_is_async - is %current an async worker task?
+ *
+ * Returns %true if %current is an async worker task.
+ */
+bool current_is_async(void)
+{
+	struct worker *worker = current_wq_worker();
+
+	return worker && worker->current_func == async_run_entry_fn;
+}

kernel/workqueue_internal.h

@@ -8,6 +8,7 @@
 #define _KERNEL_WORKQUEUE_INTERNAL_H
 
 #include <linux/workqueue.h>
+#include <linux/kthread.h>
 
 struct global_cwq;
 struct worker_pool;
@@ -44,6 +45,16 @@ struct worker {
 	struct workqueue_struct	*rescue_wq;	/* I: the workqueue to rescue */
 };
 
+/**
+ * current_wq_worker - return struct worker if %current is a workqueue worker
+ */
+static inline struct worker *current_wq_worker(void)
+{
+	if (current->flags & PF_WQ_WORKER)
+		return kthread_data(current);
+	return NULL;
+}
+
 /*
  * Scheduler hooks for concurrency managed workqueue.  Only to be used from
  * sched.c and workqueue.c.