[PATCH 1/2] kernel: split task_work_add() into two separate helpers

[Jens Axboe <axboe@kernel.dk>]

Some callers may need to make signaling decisions based on the state
of the targeted task, and that can only safely be done post adding
the task_work to the task. Split task_work_add() into:

__task_work_add()	- adds the work item
__task_work_notify()	- sends the notification

No functional changes in this patch.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org # v5.7+
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/task_work.h | 19 ++++++++++++++++
 kernel/task_work.c        | 48 +++++++++++++++++++++------------------
 2 files changed, 45 insertions(+), 22 deletions(-)

diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 0fb93aafa478..7abbd8df5e13 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -5,6 +5,8 @@
 #include <linux/list.h>
 #include <linux/sched.h>
 
+extern struct callback_head work_exited;
+
 typedef void (*task_work_func_t)(struct callback_head *);
 
 static inline void
@@ -13,6 +15,21 @@ init_task_work(struct callback_head *twork, task_work_func_t func)
 	twork->func = func;
 }
 
+static inline int __task_work_add(struct task_struct *task,
+				  struct callback_head *work)
+{
+	struct callback_head *head;
+
+	do {
+		head = READ_ONCE(task->task_works);
+		if (unlikely(head == &work_exited))
+			return -ESRCH;
+		work->next = head;
+	} while (cmpxchg(&task->task_works, head, work) != head);
+
+	return 0;
+}
+
 #define TWA_RESUME	1
 #define TWA_SIGNAL	2
 int task_work_add(struct task_struct *task, struct callback_head *twork, int);
@@ -20,6 +37,8 @@ int task_work_add(struct task_struct *task, struct callback_head *twork, int);
 struct callback_head *task_work_cancel(struct task_struct *, task_work_func_t);
 void task_work_run(void);
 
+void __task_work_notify(struct task_struct *task, int notify);
+
 static inline void exit_task_work(struct task_struct *task)
 {
 	task_work_run();
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 5c0848ca1287..9bde81481984 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -3,7 +3,27 @@
 #include <linux/task_work.h>
 #include <linux/tracehook.h>
 
-static struct callback_head work_exited; /* all we need is ->next == NULL */
+struct callback_head work_exited = {
+	.next = NULL	/* all we need is ->next == NULL */
+};
+
+void __task_work_notify(struct task_struct *task, int notify)
+{
+	unsigned long flags;
+
+	switch (notify) {
+	case TWA_RESUME:
+		set_notify_resume(task);
+		break;
+	case TWA_SIGNAL:
+		if (lock_task_sighand(task, &flags)) {
+			task->jobctl |= JOBCTL_TASK_WORK;
+			signal_wake_up(task, 0);
+			unlock_task_sighand(task, &flags);
+		}
+		break;
+	}
+}
 
 /**
  * task_work_add - ask the @task to execute @work->func()
@@ -27,29 +47,13 @@ static struct callback_head work_exited; /* all we need is ->next == NULL */
 int
 task_work_add(struct task_struct *task, struct callback_head *work, int notify)
 {
-	struct callback_head *head;
-	unsigned long flags;
+	int ret;
 
-	do {
-		head = READ_ONCE(task->task_works);
-		if (unlikely(head == &work_exited))
-			return -ESRCH;
-		work->next = head;
-	} while (cmpxchg(&task->task_works, head, work) != head);
-
-	switch (notify) {
-	case TWA_RESUME:
-		set_notify_resume(task);
-		break;
-	case TWA_SIGNAL:
-		if (lock_task_sighand(task, &flags)) {
-			task->jobctl |= JOBCTL_TASK_WORK;
-			signal_wake_up(task, 0);
-			unlock_task_sighand(task, &flags);
-		}
-		break;
-	}
+	ret = __task_work_add(task, work);
+	if (unlikely(ret))
+		return ret;
 
+	__task_work_notify(task, notify);
 	return 0;
 }
 
-- 
2.28.0