linux-kernel - Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil

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Date:   Tue, 18 Dec 2018 14:14:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Yongji Xie <elohimes@...il.com>, mingo@...hat.com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        Xie Yongji <xieyongji@...du.com>, zhangyu31@...du.com,
        liuqi16@...du.com, yuanlinsi01@...du.com, nixun@...du.com,
        lilin24@...du.com, longman@...hat.com,
        andrea.parri@...rulasolutions.com
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the
 reader waiter to nil

On Tue, Dec 18, 2018 at 02:10:31PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 17, 2018 at 12:53:10PM -0800, Davidlohr Bueso wrote:

> > So how about the following to reduce some of the performance penalty (at
> > the cost of more complexity)?
> 
> I'd rather do it like so, except I'm still conflicted on the naming.

> +void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> +{
> +	if (__wake_q_add(head, task))
> +		get_task_struct(task);
> +}
> +
> +void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task)
> +{
> +	if (!__wake_a_add(head, task))
> +		put_task_struct(task);
>  }

That is, in the one case it has internal reference counting, in the
other case it consumes the reference counting.