From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v6 05/11] pvqspinlock,
 x86: Allow unfair spinlock in a PV guest
Date: Fri, 14 Mar 2014 09:48:09 +0100
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To: Peter Zijlstra <peterz@infradead.org>, Waiman Long <waiman.long@hp.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, Gleb Natapov <gleb@redhat.com>, 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, kvm@vger.kernel.org, x86@kernel.org, Alok Kataria <akataria@vmware.com>, 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>, Steven Rostedt <rostedt@goodmis.org>, Chris Wright <chrisw@sous-sol.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Aswin Chandramouleeswaran <aswin@hp.com>, Oleg Nesterov <oleg@redhat.com>
List-Id: xen-devel@lists.xenproject.org

Il 14/03/2014 09:30, Peter Zijlstra ha scritto:
> Take the situation of 3 (v)CPUs where cpu0 holds the lock but is
> preempted. cpu1 queues, cpu2 queues. Then cpu1 gets preempted, after
> which cpu0 gets back online.
>
> The simple test-and-set lock will now let cpu2 acquire. Your queue
> however will just sit there spinning, waiting for cpu1 to come back from
> holiday.
>
> I think you're way over engineering this. Just do the simple
> test-and-set lock for virt && !paravirt (as I think Paolo Bonzini
> suggested RHEL6 already does).

Exactly.

Paolo