linux-kernel - Re: [PATCH v3 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY

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Date:   Tue, 15 May 2018 20:02:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED
 flag

On Tue, May 15, 2018 at 01:38:03PM -0400, Waiman Long wrote:
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index e795908..a27dbb4 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -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;
>  	}
>  
> @@ -382,8 +379,10 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
>  {
>  	struct task_struct *owner = READ_ONCE(sem->owner);
>  
> -	if (!rwsem_owner_is_writer(owner))
> -		goto out;
> +	if (!owner)
> +		return true;
> +	else if (!is_rwsem_owner_spinnable(owner))
> +		return false;

s/else //

>  	rcu_read_lock();
>  	while (sem->owner == owner) {
> @@ -408,12 +407,12 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
>  		cpu_relax();
>  	}
>  	rcu_read_unlock();
> -out:
> +
>  	/*
>  	 * If there is a new owner or the owner is not set, we continue
>  	 * spinning.
>  	 */
> -	return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
> +	return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
>  }

The above two cases have explicit owner tests, this one doesn't.

Now I know it works, but maybe:

	return !owner || is_rwsem_owner_spinnable(owner);

is easier to read... dunno.