[PATCH 1/6] task_work: Provide means to check if a work is queued

[Frederic Weisbecker <frederic@kernel.org>]

Some task work users implement their own ways to know if a callback is
already queued on the current task while fiddling with the callback
head internals.

Provide instead a consolidated API to serve this very purpose.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 include/linux/task_work.h | 12 ++++++++++++
 kernel/task_work.c        |  9 +++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 0646804860ff..31caf12c1313 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -5,12 +5,15 @@
 #include <linux/list.h>
 #include <linux/sched.h>
 
+#define TASK_WORK_DEQUEUED	((void *) -1UL)
+
 typedef void (*task_work_func_t)(struct callback_head *);
 
 static inline void
 init_task_work(struct callback_head *twork, task_work_func_t func)
 {
 	twork->func = func;
+	twork->next = TASK_WORK_DEQUEUED;
 }
 
 enum task_work_notify_mode {
@@ -26,6 +29,15 @@ static inline bool task_work_pending(struct task_struct *task)
 	return READ_ONCE(task->task_works);
 }
 
+/*
+ * Check if a work is queued. Beware: this is inherently racy if the work can
+ * be queued elsewhere than the current task.
+ */
+static inline bool task_work_queued(struct callback_head *twork)
+{
+	return twork->next != TASK_WORK_DEQUEUED;
+}
+
 int task_work_add(struct task_struct *task, struct callback_head *twork,
 			enum task_work_notify_mode mode);
 
diff --git a/kernel/task_work.c b/kernel/task_work.c
index d1efec571a4a..56718cb824d9 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -67,8 +67,10 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
 
 	head = READ_ONCE(task->task_works);
 	do {
-		if (unlikely(head == &work_exited))
+		if (unlikely(head == &work_exited)) {
+			work->next = TASK_WORK_DEQUEUED;
 			return -ESRCH;
+		}
 		work->next = head;
 	} while (!try_cmpxchg(&task->task_works, &head, work));
 
@@ -129,8 +131,10 @@ task_work_cancel_match(struct task_struct *task,
 		if (!match(work, data)) {
 			pprev = &work->next;
 			work = READ_ONCE(*pprev);
-		} else if (try_cmpxchg(pprev, &work, work->next))
+		} else if (try_cmpxchg(pprev, &work, work->next)) {
+			work->next = TASK_WORK_DEQUEUED;
 			break;
+		}
 	}
 	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
 
@@ -224,6 +228,7 @@ void task_work_run(void)
 
 		do {
 			next = work->next;
+			work->next = TASK_WORK_DEQUEUED;
 			work->func(work);
 			work = next;
 			cond_resched();
-- 
2.48.1