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

From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Yongji Xie <elohimes@gmail.com>,
	mingo@redhat.com, will.deacon@arm.com,
	linux-kernel@vger.kernel.org, Xie Yongji <xieyongji@baidu.com>,
	zhangyu31@baidu.com, liuqi16@baidu.com, yuanlinsi01@baidu.com,
	nixun@baidu.com, lilin24@baidu.com, longman@redhat.com,
	andrea.parri@amarulasolutions.com
Subject: Re: [RFC] locking/rwsem: Avoid issuing wakeup before setting the reader waiter to nil
Date: Tue, 18 Dec 2018 14:14:19 +0100	[thread overview]
Message-ID: <20181218131419.GD16284@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20181218131031.GJ15430@hirez.programming.kicks-ass.net>

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.