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

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Date:	Fri, 09 May 2014 20:49:50 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 05/08/2014 02:57 PM, Peter Zijlstra wrote:
> 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.

That is true. I will make the change to get rid of the pointer thing.

As for the separate trylock_pending function, my original goal was to 
have a better delineation of different portions of the code.  Given the 
fact that I broke up the slowpath function into 2 in a later patch, I 
may not really need to separate it out. I will pull it back in the next 
version.

-Longman
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