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

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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>
This commit is contained in:
Andrea Parri 2020-01-22 19:39:52 +01:00 committed by Tejun Heo
parent 0bf999f9c5
commit dbb92f8864
1 changed files with 16 additions and 0 deletions
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16
include/linux/workqueue.h
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@ -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)
{