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Re: [Xen-devel] [PATCH v5 1/8] qspinlock: Introducing a 4-byte queue spinlock implementation
- To: Waiman Long <Waiman.Long@xxxxxx>
- From: Oleg Nesterov <oleg@xxxxxxxxxx>
- Date: Sun, 2 Mar 2014 14:31:40 +0100
- Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, Andi Kleen <andi@xxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Michel Lespinasse <walken@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, linux-arch@xxxxxxxxxxxxxxx, Gleb Natapov <gleb@xxxxxxxxxx>, x86@xxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Scott J Norton <scott.norton@xxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, Alok Kataria <akataria@xxxxxxxxxx>, Aswin Chandramouleeswaran <aswin@xxxxxx>, Chegu Vinod <chegu_vinod@xxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, David Vrabel <david.vrabel@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Sun, 02 Mar 2014 14:26:08 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
Forgot to ask...
On 02/26, Waiman Long wrote:
>
> +notify_next:
> + /*
> + * Wait, if needed, until the next one in queue set up the next field
> + */
> + while (!(next = ACCESS_ONCE(node->next)))
> + arch_mutex_cpu_relax();
> + /*
> + * The next one in queue is now at the head
> + */
> + smp_store_release(&next->wait, false);
Do we really need smp_store_release()? It seems that we can rely on the
control dependency here. And afaics there is no need to serialise this
store with other changes in *lock, plus they all have mb's anyway.
Oleg.
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