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Subject[PATCH v2] workqueue: Document (some) memory-ordering properties of {queue,schedule}_work()
Date
It's desirable to be able to rely on the following property:  All stores
preceding (in program order) a call to a successful queue_work() will be
visible from the CPU which will execute the queued work by the time such
work executes, e.g.,

{ x is initially 0 }

CPU0 CPU1

WRITE_ONCE(x, 1); [ "work" is being executed ]
r0 = queue_work(wq, work); r1 = READ_ONCE(x);

Forbids: r0 == true && r1 == 0

The current implementation of queue_work() provides such memory-ordering
property:

- In __queue_work(), the ->lock spinlock is acquired.

- On the other side, in worker_thread(), this same ->lock is held
when dequeueing work.

So the locking ordering makes things work out.

Add this property to the DocBook headers of {queue,schedule}_work().

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
---
Changes since v1 [1]:
- fix typo (Randy Dunlap)
- add Acked-by: tag (Paul E. McKenney)

[1] https://lkml.kernel.org/r/20200118215820.7646-1-parri.andrea@gmail.com

include/linux/workqueue.h | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 4261d1c6e87b1..e48554e6526c0 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -487,6 +487,19 @@ extern void wq_worker_comm(char *buf, size_t size, struct task_struct *task);
*
* We queue the work to the CPU on which it was submitted, but if the CPU dies
* it can be processed by another CPU.
+ *
+ * Memory-ordering properties: If it returns %true, guarantees that all stores
+ * preceding the call to queue_work() in the program order will be visible from
+ * the CPU which will execute @work by the time such work executes, e.g.,
+ *
+ * { x is initially 0 }
+ *
+ * CPU0 CPU1
+ *
+ * WRITE_ONCE(x, 1); [ @work is being executed ]
+ * r0 = queue_work(wq, work); r1 = READ_ONCE(x);
+ *
+ * Forbids: r0 == true && r1 == 0
*/
static inline bool queue_work(struct workqueue_struct *wq,
struct work_struct *work)
@@ -546,6 +559,9 @@ static inline bool schedule_work_on(int cpu, struct work_struct *work)
* This puts a job in the kernel-global workqueue if it was not already
* queued and leaves it in the same position on the kernel-global
* workqueue otherwise.
+ *
+ * Shares the same memory-ordering properties of queue_work(), cf. the
+ * DocBook header of queue_work().
*/
static inline bool schedule_work(struct work_struct *work)
{
--
2.24.0
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