Date: Thu, 17 Apr 2014 17:49:02 +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: <20140417154902.GO11096@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>
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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!
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