From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH v3 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD Date: Mon, 31 Jan 2011 15:22:43 +0200 Message-ID: <4D46B7A3.3000106@redhat.com> References: <4D417F1F.7020302@siemens.com> <4D418230.1010801@siemens.com> <4D4688EB.30408@redhat.com> <4D469C87.3080909@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "kvm@vger.kernel.org" , "qemu-devel@nongnu.org" , Stefan Hajnoczi To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:1027 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752683Ab1AaNWu (ORCPT ); Mon, 31 Jan 2011 08:22:50 -0500 In-Reply-To: <4D469C87.3080909@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On 01/31/2011 01:27 PM, Jan Kiszka wrote: > On 2011-01-31 11:03, Avi Kivity wrote: > > On 01/27/2011 04:33 PM, Jan Kiszka wrote: > >> Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between > >> checking for exit_request on vcpu entry and timer signals arriving > >> before KVM starts to catch them. Plug it by blocking both timer related > >> signals also on !CONFIG_IOTHREAD and process those via signalfd. > >> > >> As this fix depends on real signalfd support (otherwise the timer > >> signals only kick the compat helper thread, and the main thread hangs), > >> we need to detect the invalid constellation and abort configure. > >> > >> Signed-off-by: Jan Kiszka > >> CC: Stefan Hajnoczi > >> --- > >> > >> I don't want to invest that much into !IOTHREAD anymore, so let's see if > >> the proposed catch&abort is acceptable. > >> > > > > I don't understand the dependency on signalfd. The normal way of doing > > things, either waiting for the signal in sigtimedwait() or in > > ioctl(KVM_RUN), works with SIGALRM just fine. > > And how would you be kicked out of the select() call if it is waiting > with a timeout? We only have a single thread here. If we use signalfd() (either kernel provided or thread+pipe), we kick out of select by select()ing it (though I don't see how it works without an iothread, since an fd can't stop a vcpu unless you enable SIGIO on it, which is silly for signalfd) If you leave it as a naked signal, then it can break out of either pselect() or vcpu. Since the goal is to drop !CONFIG_IOTHREAD, the first path seems better, I just don't understand the problem with emulated signalfd(). -- error compiling committee.c: too many arguments to function