From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=36967 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Pjscy-0002c5-6l for qemu-devel@nongnu.org; Mon, 31 Jan 2011 07:13:05 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Pjscw-0003OQ-JD for qemu-devel@nongnu.org; Mon, 31 Jan 2011 07:13:04 -0500 Received: from mail-gx0-f173.google.com ([209.85.161.173]:47708) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Pjscw-0003O8-Gc for qemu-devel@nongnu.org; Mon, 31 Jan 2011 07:13:02 -0500 Received: by gxk28 with SMTP id 28so2205718gxk.4 for ; Mon, 31 Jan 2011 04:13:01 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <4D469C87.3080909@siemens.com> References: <4D417F1F.7020302@siemens.com> <4D418230.1010801@siemens.com> <4D4688EB.30408@redhat.com> <4D469C87.3080909@siemens.com> Date: Mon, 31 Jan 2011 12:13:01 +0000 Message-ID: Subject: Re: [Qemu-devel] Re: [PATCH v3 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Marcelo Tosatti , Stefan Hajnoczi , Avi Kivity , "kvm@vger.kernel.org" , "qemu-devel@nongnu.org" On Mon, Jan 31, 2011 at 11:27 AM, 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 i= f >>> the proposed catch&abort is acceptable. >>> >> >> I don't understand the dependency on signalfd. =A0The normal way of doin= g >> 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. > > The only alternative is Stefan's original proposal. But that required > fiddling with the signal mask twice per KVM_RUN. I think my original patch messed with the sigmask in the wrong place, as you mentioned doing it twice per KVM_RUN isn't a good idea. I wonder if we can enable SIGALRM only in blocking calls and guest code execution but without signalfd. It might be possible, I don't see an immediate problem with doing that, we might have to use pselect(2) or similar in a few places. Stefan