linux-kernel - Re: [PATCH v4 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY

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Date:   Wed, 16 May 2018 05:19:47 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Oleg Nesterov <oleg@...hat.com>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v4 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED
 flag

On Tue, May 15, 2018 at 05:49:50PM -0400, Waiman Long wrote:
> @@ -357,11 +357,8 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>  
>  	rcu_read_lock();
>  	owner = READ_ONCE(sem->owner);
> -	if (!rwsem_owner_is_writer(owner)) {
> -		/*
> -		 * Don't spin if the rwsem is readers owned.
> -		 */
> -		ret = !rwsem_owner_is_reader(owner);
> +	if (!owner || !is_rwsem_owner_spinnable(owner)) {
> +		ret = !owner;	/* !owner is spinnable */
>  		goto done;
>  	}

This is confusingly written.  I think you mean ...

	if (!owner)
		goto done;
	if (!is_rwsem_owner_spinnable(owner)) {
		ret = false;
		goto done;
	}