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SubjectRe: [PATCH] workqueue: Document (some) memory-ordering properties of {queue,schedule}_work()
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Hi,

On 1/18/20 1:58 PM, Andrea Parri wrote:
> 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>
> ---
> include/linux/workqueue.h | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index 4261d1c6e87b1..4fef6c38b0536 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(), c.f., the

nit: cf. the

> + * DocBook header of queue_work().
> */
> static inline bool schedule_work(struct work_struct *work)
> {
>


--
~Randy

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