Re: [PATCH v3] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write

[Peter Zijlstra]

On Fri, Apr 24, 2015 at 01:54:29PM -0400, Waiman Long wrote:
> @@ -478,7 +515,40 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
>  {
>  	unsigned long flags;
>  
> -	raw_spin_lock_irqsave(&sem->wait_lock, flags);
> +	/*
> +	 * If a spinner is present, it is not necessary to do the wakeup.
> +	 * Try to do wakeup only if the trylock succeeds to minimize
> +	 * spinlock contention which may introduce too much delay in the
> +	 * unlock operation.
> +	 *
> +	 *    spinning writer		up_write/up_read caller
> +	 *    ---------------		-----------------------
> +	 * [S]   osq_unlock()		[L]   osq
> +	 *	 MB			      MB
> +	 * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
> +	 *
> +	 * Here, it is important to make sure that there won't be a missed
> +	 * wakeup while the rwsem is free and the only spinning writer goes
> +	 * to sleep without taking the rwsem. In case the spinning writer is
> +	 * just going to break out of the waiting loop, it will still do a
> +	 * trylock in rwsem_down_write_failed() before sleeping. IOW, if
> +	 * rwsem_has_spinner() is true, it will  guarantee at least one
> +	 * trylock attempt on the rwsem.
> +	 */
> +	if (!rwsem_has_spinner(sem)) {
> +		raw_spin_lock_irqsave(&sem->wait_lock, flags);
> +	} else {
> +		/*
> +		 * rwsem_has_spinner() is an atomic read while spin_trylock
> +		 * does not guarantee a full memory barrier. Insert a memory
> +		 * barrier here to make sure that wait_lock isn't read until
> +		 * after osq.
> +		 * Note: smp_rmb__after_atomic() should be used if available.
> +		 */
> +		smp_mb__after_atomic();
> +		if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> +			return sem;
> +	}
>  
>  	/* do nothing if list empty */
>  	if (!list_empty(&sem->wait_list))

To me it makes more sense to reverse these two branches (identical code
wise of course) and put the special case first.

Alternatively we could also do something like the below, which to my
eyes looks a little better still, but I don't care too much.

	if (rwsem_has_spinner(sem)) {
		/*
		 * comment ...
		 */
		 smp_rmb();
		 if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
			return sem;
		 goto locked;
	}

	raw_spin_lock_irqsave(&sem->wait_lock, flags);
locked:
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