linux-kernel - Re: [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait

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Date:   Mon, 21 Nov 2016 13:55:35 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     mingo@...nel.org, peterz@...radead.org, john.stultz@...aro.org,
        dimitrysh@...gle.com, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues
        with swait

On 11/18, Davidlohr Bueso wrote:
>
> @@ -12,7 +12,7 @@ struct percpu_rw_semaphore {
>  	struct rcu_sync		rss;
>  	unsigned int __percpu	*read_count;
>  	struct rw_semaphore	rw_sem;
> -	wait_queue_head_t	writer;
> +	struct swait_queue_head	writer;

I won't argue, but even swait_queue_head is overkill in this case.

We can just add "struct task_struct *writer" into percpu_rw_semaphore,

__percpu_up_read:

	rcu_read_lock();
	writer = task_rcu_dereference(&sem->writer);
	if (writer)
		wake_up_process(writer);
	rcu_read_unlock();

percpu_down_write() can set sem->writer == current and do the simple
while-not-condition-schedule() loop.

But this probably needs a couple of new helpers, and probably they
can have more users.

Oleg.