From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=51698 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OOFYS-0002hR-Ba for qemu-devel@nongnu.org; Mon, 14 Jun 2010 15:42:45 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OOFYQ-0004gK-W6 for qemu-devel@nongnu.org; Mon, 14 Jun 2010 15:42:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:23478) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OOFYQ-0004gB-NL for qemu-devel@nongnu.org; Mon, 14 Jun 2010 15:42:42 -0400 Date: Mon, 14 Jun 2010 16:42:37 -0300 From: Glauber Costa Subject: Re: [Qemu-devel] [PATCH] stop cpus before forking. Message-ID: <20100614194237.GD19104@mothafucka.localdomain> References: <1276543644-32689-1-git-send-email-glommer@redhat.com> <4C1683EC.3010609@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4C1683EC.3010609@codemonkey.ws> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: aliguori@us.ibm.com, qemu-devel@nongnu.org, avi@redhat.com On Mon, Jun 14, 2010 at 02:33:00PM -0500, Anthony Liguori wrote: > On 06/14/2010 02:27 PM, Glauber Costa wrote: > >This patch fixes a bug that happens with kvm, irqchip-in-kernel, > >while adding a netdev. Despite the situations of reproduction being > >specific to kvm, I believe this fix is pretty generic, and fits here. > >Specially if we ever want to have our own irqchip in kernel too. > > > >The problem happens after the fork system call, and although it is not > >100 % reproduceable, happens pretty often. After fork, the memory where > >the apic is mapped is present in both processes. It ends up confusing > >the vcpus somewhere in the irq<-> ack path, and qemu hangs, with no > >irqs being delivered at all from that point on. > > > >Making sure the vcpus are stopped before forking makes the problem go > >away. Besides, this is a pretty unfrequent operation, which already hangs > >the io-thread for a while. So it should not hurt performance. > > > >Signed-off-by: Glauber Costa > > This doesn't make very much sense to me but smells like a kernel bug to me. My interpretation is that by doing that, we make sure no in-flight requests are happening. Actually, a sleep(x), with x sufficiently big is enough to make this problem go away, but that is too hacky. I do agree that this is most likely a kernel bug. But as with any other kernel bugs, I believe this is a easy workaround to have things working even in older kernels until we fix it. > > Even if it isn't, I can't rationalize why stopping the vm like this > is enough to fix such a problem. Is the problem that the KVM VCPU > threads get duplicated while potentially running or something like > that? I doubt fork is duplicating the vcpu threads. More than that, this bug does not happen with userspace irqchip. So I believe that either irq request or the ack itself is reaching the wrong process, forever stalling the apic.