From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [patch 2/4] KVM: move coalesced_mmio locking to its own device Date: Mon, 1 Jun 2009 18:23:30 -0300 Message-ID: <20090601212330.GA6834@amt.cnet> References: <20090528044552.151652861@localhost.localdomain> <20090528044808.205238362@localhost.localdomain> <4A2274AC.5050303@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from mx2.redhat.com ([66.187.237.31]:49150 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751364AbZFAVXm (ORCPT ); Mon, 1 Jun 2009 17:23:42 -0400 Received: from int-mx2.corp.redhat.com (int-mx2.corp.redhat.com [172.16.27.26]) by mx2.redhat.com (8.13.8/8.13.8) with ESMTP id n51LNi7u026574 for ; Mon, 1 Jun 2009 17:23:44 -0400 Content-Disposition: inline In-Reply-To: <4A2274AC.5050303@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Sun, May 31, 2009 at 03:14:36PM +0300, Avi Kivity wrote: > Marcelo Tosatti wrote: >> Move coalesced_mmio locking to its own device, instead of relying on >> kvm->lock. >> >> Signed-off-by: Marcelo Tosatti >> >> Index: kvm-irqlock/virt/kvm/coalesced_mmio.c >> =================================================================== >> --- kvm-irqlock.orig/virt/kvm/coalesced_mmio.c >> +++ kvm-irqlock/virt/kvm/coalesced_mmio.c >> @@ -26,9 +26,7 @@ static int coalesced_mmio_in_range(struc >> if (!is_write) >> return 0; >> - /* kvm->lock is taken by the caller and must be not released before >> - * dev.read/write >> - */ >> + spin_lock(&dev->lock); >> > > This unbalanced locking is still very displeasing. At a minimum you > need a sparse annotation to indicate it. > > But I think it really indicates a problem with the io_device API. > > Potential solutions: > - fold in_range() into ->write and ->read. Make those functions > responsible for both determining whether they can handle the range and > performing the I/O. > - have a separate rwlock for the device list. IMO the problem is the coalesced_mmio device. The unbalanced locking is a result of the abuse of the in_range() and read/write() methods. Normally you'd expect parallel accesses to in_range() to be allowed, since its just checking whether (aha) the access is in range, returning a pointer to the device if positive. Now read/write() are the ones who need serialization, since they touch the device internal state. coalesced_mmio abuses in_range() to do more things than it should. Ideally we should fix coalesced_mmio, but i'm not going to do that now (sorry, not confident in changing it without seeing go through intense torture testing). That said, is sparse annotation enough the convince you?