[PATCH 2/4] task_work: don't run task_work if task_work_exited is queued

[Jens Axboe <axboe@kernel.dk>]

If task_work has already been run on task exit, we don't always know
if it's safe to run again. Check for task_work_exited in the
task_work_pending() helper. This makes it less fragile in calling
from the exit files path, for example.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/task_work.h | 4 +++-
 kernel/task_work.c        | 8 ++++----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 54c911bbf754..24f977a8fc35 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -7,6 +7,8 @@
 
 typedef void (*task_work_func_t)(struct callback_head *);
 
+extern struct callback_head task_work_exited;
+
 static inline void
 init_task_work(struct callback_head *twork, task_work_func_t func)
 {
@@ -19,7 +21,7 @@ void __task_work_run(void);
 
 static inline bool task_work_pending(void)
 {
-	return current->task_works;
+	return current->task_works && current->task_works != &task_work_exited;
 }
 
 static inline void task_work_run(void)
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 9620333423a3..d6a8b4ab4858 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -3,7 +3,7 @@
 #include <linux/task_work.h>
 #include <linux/tracehook.h>
 
-static struct callback_head work_exited; /* all we need is ->next == NULL */
+struct callback_head task_work_exited = { };
 
 /**
  * task_work_add - ask the @task to execute @work->func()
@@ -31,7 +31,7 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
 
 	do {
 		head = READ_ONCE(task->task_works);
-		if (unlikely(head == &work_exited))
+		if (unlikely(head == &task_work_exited))
 			return -ESRCH;
 		work->next = head;
 	} while (cmpxchg(&task->task_works, head, work) != head);
@@ -95,14 +95,14 @@ void __task_work_run(void)
 	for (;;) {
 		/*
 		 * work->func() can do task_work_add(), do not set
-		 * work_exited unless the list is empty.
+		 * task_work_exited unless the list is empty.
 		 */
 		do {
 			head = NULL;
 			work = READ_ONCE(task->task_works);
 			if (!work) {
 				if (task->flags & PF_EXITING)
-					head = &work_exited;
+					head = &task_work_exited;
 				else
 					break;
 			}
-- 
2.26.0