linux-kernel - Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word

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Date:	Thu, 17 Apr 2014 17:28:17 -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>,
	"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 v9 04/19] qspinlock: Extract out the exchange of tail
 code word

On 04/17/2014 11:49 AM, Peter Zijlstra wrote:
> On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote:
>> @@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>   	node->next = NULL;
>>
>>   	/*
>> +	 * We touched a (possibly) cold cacheline; attempt the trylock once
>> +	 * more in the hope someone let go while we weren't watching as long
>> +	 * as no one was queuing.
>>   	 */
>> +	if (!(val&  _Q_TAIL_MASK)&&  queue_spin_trylock(lock))
>> +		goto release;
> But you just did a potentially very expensive op; @val isn't
> representative anymore!

That is not true. I pass in a pointer to val to trylock_pending() (the 
pointer thing) so that it will store the latest value that it reads from 
the lock back into val. I did miss one in the PV qspinlock exit loop. I 
will add it back when I do the next version.

-Longman
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