From mboxrd@z Thu Jan  1 00:00:00 1970
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v5 2/8] qspinlock,
	x86: Enable x86-64 to use queue	spinlock
Date: Sun, 2 Mar 2014 15:10:58 +0100
Message-ID: <20140302141058.GA15629@redhat.com>
References: <1393475567-4453-1-git-send-email-Waiman.Long@hp.com>
	<1393475567-4453-3-git-send-email-Waiman.Long@hp.com>
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To: Waiman Long <Waiman.Long@hp.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, virtualization@lists.linux-foundation.org, Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>, Michel Lespinasse <walken@google.com>, Thomas Gleixner <tglx@linutronix.de>, linux-arch@vger.kernel.org, Gleb Natapov <gleb@redhat.com>, x86@kernel.org, Ingo Molnar <mingo@redhat.com>, xen-devel@lists.xenproject.org, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>, Scott J Norton <scott.norton@hp.com>, Rusty Russell <rusty@rustcorp.com.au>, Steven Rostedt <rostedt@goodmis.org>, Chris Wright <chrisw@sous-sol.org>, Alok Kataria <akataria@vmware.com>, Aswin Chandramouleeswaran <aswin@hp.com>, Chegu Vinod <chegu_vinod@hp.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, linux-kernel
List-Id: linux-arch.vger.kernel.org

On 02/26, Waiman Long wrote:
>
> +#define _ARCH_SUPPORTS_ATOMIC_8_16_BITS_OPS
> +
> +/*
> + * x86-64 specific queue spinlock union structure
> + */
> +union arch_qspinlock {
> +	struct qspinlock slock;
> +	u8		 lock;	/* Lock bit	*/
> +};

And this enables the optimized version of queue_spin_setlock().

But why does it check ACCESS_ONCE(qlock->lock) == 0 ? This is called
right after queue_get_lock_qcode() returns 0, this locked should be
likely unlocked.

Oleg.