linux-kernel - Re: [PATCH] workqueue: Document (some) memory-ordering properties of {queue,schedule}

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Message-ID: <20200120020235.GA8126@paulmck-ThinkPad-P72>
Date:   Sun, 19 Jan 2020 18:02:35 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] workqueue: Document (some) memory-ordering properties of
 {queue,schedule}_work()

On Sat, Jan 18, 2020 at 10:58:20PM +0100, 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@...nel.org>
> Signed-off-by: Andrea Parri <parri.andrea@...il.com>

Acked-by: Paul E. McKenney <paulmck@...nel.org>

An alternative to Randy's suggestion of dropping the comma following
the "cf." is to just drop that whole phrase.  I will let you and Randy
work that one out, though.  ;-)

> ---
>  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
> + * DocBook header of queue_work().
>   */
>  static inline bool schedule_work(struct work_struct *work)
>  {
> -- 
> 2.24.0
>