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

[Peter Zijlstra]

On Fri, Apr 17, 2015 at 10:03:18PM -0400, Waiman Long wrote:
> @@ -478,7 +515,28 @@ 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.
> +	 *
> +	 * 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.

successful trylock? I think we're having 'issues' on if failed trylocks
(and cmpxchg) imply full barriers.

> +	 *
> +	 *    spinning writer
> +	 *    ---------------
> +	 * [S]   osq_unlock()
> +	 *	 MB
> +	 * [RmW] rwsem_try_write_lock()
> +	 */

Ordering comes in pairs, this is incomplete.

> +	if (!rwsem_has_spinner(sem))
> +		raw_spin_lock_irqsave(&sem->wait_lock, flags);
> +	else if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> +		return sem;
>  
>  	/* do nothing if list empty */
>  	if (!list_empty(&sem->wait_list))
> -- 
> 1.7.1
> 
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