workqueue: Document (some) memory-ordering properties of {queue,schedule}_work()

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>
Signed-off-by: Tejun Heo <tj@kernel.org>

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)
 {