From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH] kvm: Drop obsolete cpu_get/put in make_all_cpus_request
Date: Tue, 21 Jul 2009 14:10:25 -0300
Message-ID: <20090721171025.GA6959@amt.cnet>
References: <4A643924.6060908@siemens.com> <20090721000054.GB15189@amt.cnet> <4A657B28.5030006@siemens.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: kvm-devel <kvm@vger.kernel.org>
To: Jan Kiszka <jan.kiszka@siemens.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx2.redhat.com ([66.187.237.31]:50557 "EHLO mx2.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1755753AbZGURLK (ORCPT <rfc822;kvm@vger.kernel.org>);
	Tue, 21 Jul 2009 13:11:10 -0400
Content-Disposition: inline
In-Reply-To: <4A657B28.5030006@siemens.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On Tue, Jul 21, 2009 at 10:24:08AM +0200, Jan Kiszka wrote:
> Marcelo Tosatti wrote:
> > Jan,
> > 
> > This was suggested but we thought it might be safer to keep the
> > get_cpu/put_cpu pair in case -rt kernels require it (which might be
> > bullshit, but nobody verified).
> 
> -rt stumbles over both patterns (that's why I stumbled over it in the
> first place: get_cpu disables preemption, but spin_lock is a sleeping
> lock under -rt) and actually requires requests_lock to become
> raw_spinlock_t. Reordering get_cpu and spin_lock would be another
> option, but not really a gain for both scenarios.

I see.

> So unless there is a way to make the whole critical section preemptible
> (thus migration-agnostic), I think we can micro-optimize it like this.

Can't you switch requests_lock to be raw_spinlock_t then? (or whatever
is necessary to make it -rt compatible).