[PATCH 1/6 v2] task_work: Provide means to check if a work is queued

From: Frederic Weisbecker
Date: Sun Feb 09 2025 - 17:30:33 EST


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.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
include/linux/task_work.h | 12 ++++++++++++
kernel/task_work.c | 11 +++++++++--
2 files changed, 21 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..0d7b04095753 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -56,6 +56,8 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
{
struct callback_head *head;

+ work->next = TASK_WORK_DEQUEUED;
+
if (notify == TWA_NMI_CURRENT) {
if (WARN_ON_ONCE(task != current))
return -EINVAL;
@@ -67,8 +69,10 @@ 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 +133,10 @@ 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 +230,7 @@ void task_work_run(void)

do {
next = work->next;
+ work->next = TASK_WORK_DEQUEUED;
work->func(work);
work = next;
cond_resched();
--
2.46.0