[PATCH 1/2] task_work: minor cleanups

[PATCH 1/2] task_work: minor cleanups

[Oleg Nesterov]

Trivial. Remove the unnecessary "work = NULL" initialization
and turn read_barrier_depends() into smp_read_barrier_depends()
in task_work_cancel().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/task_work.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 65bd3c9..6ee0985 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -25,7 +25,7 @@ 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 = NULL;
+	struct callback_head *work;
 	unsigned long flags;
 	/*
 	 * If cmpxchg() fails we continue without updating pprev.
@@ -35,7 +35,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
 	 */
 	raw_spin_lock_irqsave(&task->pi_lock, flags);
 	while ((work = ACCESS_ONCE(*pprev))) {
-		read_barrier_depends();
+		smp_read_barrier_depends();
 		if (work->func != func)
 			pprev = &work->next;
 		else if (cmpxchg(pprev, work, work->next) == work)
-- 
1.5.5.1

[PATCH 2/2] task_work: documentation

[Oleg Nesterov]

No functional changes, just comments.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/task_work.c |   36 ++++++++++++++++++++++++++++++++++++
 1 files changed, 36 insertions(+), 0 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 6ee0985..8727032 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -4,6 +4,23 @@
 
 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.
+ *
+ * RETURNS:
+ * 0 if succeeds or -ESRCH.
+ */
 int
 task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
 {
@@ -21,6 +38,17 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
 	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)
 {
@@ -46,6 +74,14 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
 	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;
-- 
1.5.5.1