linux-kernel - Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

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Date:	Mon, 9 May 2016 10:27:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

On Fri, May 06, 2016 at 08:20:24PM -0400, Waiman Long wrote:
> @@ -391,9 +386,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>  		 * When there's no owner, we might have preempted between the
>  		 * owner acquiring the lock and setting the owner field. If
>  		 * we're an RT task that will live-lock because we won't let
> +		 * the owner complete. We also quit if the lock is owned by
> +		 * readers.

Maybe also note why we quit on readers.

>  		 */
> +		if (rwsem_is_reader_owned(owner) ||
> +		   (!owner && (need_resched() || rt_task(current))))
>  			break;
>  
>  		/*


> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
> index 870ed9a..d7fea18 100644
> --- a/kernel/locking/rwsem.h
> +++ b/kernel/locking/rwsem.h
> @@ -1,3 +1,20 @@
> +/*
> + * The owner field of the rw_semaphore structure will be set to
> + * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
> + * the owner field when it unlocks. A reader, on the other hand, will
> + * not touch the owner field when it unlocks.
> + *
> + * In essence, the owner field now has the following 3 states:
> + *  1) 0
> + *     - lock is free or the owner hasn't set the field yet
> + *  2) RWSEM_READER_OWNED
> + *     - lock is currently or previously owned by readers (lock is free
> + *       or not set by owner yet)
> + *  3) Other non-zero value
> + *     - a writer owns the lock
> + */
> +#define RWSEM_READER_OWNED	1UL

#define RWSEM_READER_OWNED	((struct task_struct *)1UL)

> +
>  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>  static inline void rwsem_set_owner(struct rw_semaphore *sem)
>  {
> @@ -9,6 +26,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
>  	sem->owner = NULL;
>  }
>  
> +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
> +{
> +	/*
> +	 * We check the owner value first to make sure that we will only
> +	 * do a write to the rwsem cacheline when it is really necessary
> +	 * to minimize cacheline contention.
> +	 */
> +	if (sem->owner != (struct task_struct *)RWSEM_READER_OWNED)
> +		sem->owner = (struct task_struct *)RWSEM_READER_OWNED;

How much if anything did this optimization matter?

> +}
> +
> +static inline bool rwsem_is_writer_owned(struct task_struct *owner)
> +{
> +	return (unsigned long)owner > RWSEM_READER_OWNED;
> +}

Tad too clever that; what does GCC generate if you write the obvious:

	return owner && owner != RWSEM_READER_OWNER;

> +
> +static inline bool rwsem_is_reader_owned(struct task_struct *owner)
> +{
> +	return owner == (struct task_struct *)RWSEM_READER_OWNED;
> +}

So I don't particularly like these names; they read like they take a
rwsem as argument, but they don't.

Would something like: rwsem_owner_is_{reader,writer}() make more sense?

>  #else
>  static inline void rwsem_set_owner(struct rw_semaphore *sem)
>  {