Date: Fri, 18 Apr 2014 10:15:17 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-arch@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
        Paolo Bonzini <paolo.bonzini@gmail.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Rik van Riel <riel@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        David Vrabel <david.vrabel@citrix.com>,
        Oleg Nesterov <oleg@redhat.com>, Gleb Natapov <gleb@redhat.com>,
        Scott J Norton <scott.norton@hp.com>, Chegu Vinod <chegu_vinod@hp.com>
Subject: Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail
 code word
Message-ID: <20140418081517.GY11096@twins.programming.kicks-ass.net>
References: <1397747051-15401-1-git-send-email-Waiman.Long@hp.com>
 <1397747051-15401-5-git-send-email-Waiman.Long@hp.com>
 <20140417154902.GO11096@twins.programming.kicks-ass.net>
 <53504771.9070102@hp.com>
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On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote:
> 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.

But you did that read _before_ you touched a cold cacheline, that's 100s
of cycles. Whatever value you read back then is now complete nonsense.
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