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

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Message-ID: <20200120204429.GA1473@andrea>
Date:   Mon, 20 Jan 2020 21:44:29 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 Sun, Jan 19, 2020 at 06:02:35PM -0800, Paul E. McKenney wrote:
> 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>

Thanks!

>
> 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.  ;-)

Either way works for me.

I'd give Tejun and Lai some more time to review this and send a non-RFC with
your Ack and this nit fixed later this week (unless I hear some objections).

Thanks,
  Andrea