linux-kernel - Re: [PATCH v10 03/19] qspinlock: Add pending bit

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Date:	Thu, 8 May 2014 20:57:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v10 03/19] qspinlock: Add pending bit

On Wed, May 07, 2014 at 11:01:31AM -0400, Waiman Long wrote:
> +/**
> + * trylock_pending - try to acquire queue spinlock using the pending bit
> + * @lock : Pointer to queue spinlock structure
> + * @pval : Pointer to value of the queue spinlock 32-bit word
> + * Return: 1 if lock acquired, 0 otherwise
> + */
> +static inline int trylock_pending(struct qspinlock *lock, u32 *pval)

Still don't like you put it in a separate function, but you don't need
the pointer thing. Note how after you fail the trylock_pending() you
touch the second (node) cacheline.

> @@ -110,6 +184,9 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  
>  	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
>  
> +	if (trylock_pending(lock, &val))
> +		return;	/* Lock acquired */
> +
>  	node = this_cpu_ptr(&mcs_nodes[0]);
>  	idx = node->count++;
>  	tail = encode_tail(smp_processor_id(), idx);
> @@ -119,15 +196,18 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	node->next = NULL;
>  
>  	/*
> +	 * we already touched the queueing cacheline; don't bother with pending
> +	 * stuff.
> +	 *
>  	 * trylock || xchg(lock, node)
>  	 *
> -	 * 0,0 -> 0,1 ; trylock
> -	 * p,x -> n,x ; prev = xchg(lock, node)
> +	 * 0,0,0 -> 0,0,1 ; trylock
> +	 * p,y,x -> n,y,x ; prev = xchg(lock, node)
>  	 */

And any value of @val we might have had here is completely out-dated.
The only thing that makes sense it to set:

	val = 0;

Which makes us start with a trylock, alternatively we can re-read val.

>  	for (;;) {
>  		new = _Q_LOCKED_VAL;
>  		if (val)
> -			new = tail | (val & _Q_LOCKED_MASK);
> +			new = tail | (val & _Q_LOCKED_PENDING_MASK);
>  
>  		old = atomic_cmpxchg(&lock->val, val, new);
>  		if (old == val)
--
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