linux-kernel - Re: [PATCH v2] workqueue: Document (some) memory-ordering properties of {queue,schedule}

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Message-ID: <20200212210053.GA80993@mtj.thefacebook.com>
Date:   Wed, 12 Feb 2020 16:00:53 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2] workqueue: Document (some) memory-ordering properties
 of {queue,schedule}_work()

On Wed, Jan 22, 2020 at 07:39:52PM +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>

Applied to wq/for-5.7.

Thanks.

-- 
tejun