v4.19.13 snapshot.
diff --git a/kernel/task_work.c b/kernel/task_work.c
new file mode 100644
index 0000000..0fef395
--- /dev/null
+++ b/kernel/task_work.c
@@ -0,0 +1,118 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/spinlock.h>
+#include <linux/task_work.h>
+#include <linux/tracehook.h>
+
+static struct callback_head work_exited; /* all we need is ->next == NULL */
+
+/**
+ * task_work_add - ask the @task to execute @work->func()
+ * @task: the task which should run the callback
+ * @work: the callback to run
+ * @notify: send the notification if true
+ *
+ * Queue @work for task_work_run() below and notify the @task if @notify.
+ * Fails if the @task is exiting/exited and thus it can't process this @work.
+ * Otherwise @work->func() will be called when the @task returns from kernel
+ * mode or exits.
+ *
+ * This is like the signal handler which runs in kernel mode, but it doesn't
+ * try to wake up the @task.
+ *
+ * Note: there is no ordering guarantee on works queued here.
+ *
+ * RETURNS:
+ * 0 if succeeds or -ESRCH.
+ */
+int
+task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
+{
+ 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);
+
+ if (notify)
+ set_notify_resume(task);
+ return 0;
+}
+
+/**
+ * task_work_cancel - cancel a pending work added by task_work_add()
+ * @task: the task which should execute the work
+ * @func: identifies the work to remove
+ *
+ * Find the last queued pending work with ->func == @func and remove
+ * it from queue.
+ *
+ * RETURNS:
+ * The found work or NULL if not found.
+ */
+struct callback_head *
+task_work_cancel(struct task_struct *task, task_work_func_t func)
+{
+ struct callback_head **pprev = &task->task_works;
+ struct callback_head *work;
+ unsigned long flags;
+
+ if (likely(!task->task_works))
+ return NULL;
+ /*
+ * If cmpxchg() fails we continue without updating pprev.
+ * Either we raced with task_work_add() which added the
+ * new entry before this work, we will find it again. Or
+ * we raced with task_work_run(), *pprev == NULL/exited.
+ */
+ raw_spin_lock_irqsave(&task->pi_lock, flags);
+ while ((work = READ_ONCE(*pprev))) {
+ if (work->func != func)
+ pprev = &work->next;
+ else if (cmpxchg(pprev, work, work->next) == work)
+ break;
+ }
+ raw_spin_unlock_irqrestore(&task->pi_lock, flags);
+
+ return work;
+}
+
+/**
+ * task_work_run - execute the works added by task_work_add()
+ *
+ * Flush the pending works. Should be used by the core kernel code.
+ * Called before the task returns to the user-mode or stops, or when
+ * it exits. In the latter case task_work_add() can no longer add the
+ * new work after task_work_run() returns.
+ */
+void task_work_run(void)
+{
+ struct task_struct *task = current;
+ struct callback_head *work, *head, *next;
+
+ for (;;) {
+ /*
+ * work->func() can do task_work_add(), do not set
+ * work_exited unless the list is empty.
+ */
+ raw_spin_lock_irq(&task->pi_lock);
+ do {
+ work = READ_ONCE(task->task_works);
+ head = !work && (task->flags & PF_EXITING) ?
+ &work_exited : NULL;
+ } while (cmpxchg(&task->task_works, work, head) != work);
+ raw_spin_unlock_irq(&task->pi_lock);
+
+ if (!work)
+ break;
+
+ do {
+ next = work->next;
+ work->func(work);
+ work = next;
+ cond_resched();
+ } while (work);
+ }
+}