Commit dbb92f88 authored by Andrea Parri's avatar Andrea Parri Committed by Tejun Heo
Browse files

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: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarAndrea Parri <parri.andrea@gmail.com>
Acked-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 0bf999f9

include/linux/workqueue.h

+16 −0
Original line number Diff line number Diff line
@@ -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)

{