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

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Message-ID: <20181218172714.fa3zpoyqndarqgd3@linux-r8p5>
Date:   Tue, 18 Dec 2018 09:27:14 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
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, 18 Dec 2018, Peter Zijlstra wrote:
>> 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.

Yeah I like that better for an interface. Also no concurrency in head
so delaying the get() should be ok.

Thanks,
Davidlohr