From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751021AbZH0EH7 (ORCPT ); Thu, 27 Aug 2009 00:07:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750790AbZH0EH7 (ORCPT ); Thu, 27 Aug 2009 00:07:59 -0400 Received: from gate.crashing.org ([63.228.1.57]:37436 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750725AbZH0EH6 (ORCPT ); Thu, 27 Aug 2009 00:07:58 -0400 Subject: Re: Extending virtio_console to support multiple ports From: Benjamin Herrenschmidt To: Amit Shah Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, borntraeger@de.ibm.com, linux-kernel@vger.kernel.org, miltonm@bga.com, linuxppc-dev@ozlabs.org, brueckner@linux.vnet.ibm.com, alan@linux.intel.com In-Reply-To: <20090826154552.GA31910@amit-x200.redhat.com> References: <1251181044-3696-1-git-send-email-amit.shah@redhat.com> <20090826112718.GA11117@amit-x200.redhat.com> <20090826154552.GA31910@amit-x200.redhat.com> Content-Type: text/plain Date: Thu, 27 Aug 2009 14:07:03 +1000 Message-Id: <1251346023.20467.21.camel@pasglop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2009-08-26 at 21:15 +0530, Amit Shah wrote: > > > - Convert hvc's usage of spinlocks to mutexes. I've no idea how this > > will play out; I'm no expert here. But I did try doing this and so far > > it all looks OK. No lockups, lockdep warnings, nothing. I have full > > debugging enabled. But this doesn't mean it's right. > > So just to test this further I added the capability to have more than > one hvc console spawn from virtio_console, created two consoles and did > a 'cat' of a file in each of the virtio-consoles. It's been running for > half an hour now without any badness. No spew in debug logs too. > > I also checked the code in hvc_console.c that takes the spin_locks. > Nothing there that runs from (or needs to run from) interrupt context. > So the change to mutexes does seem reasonable. Also, the spinlock code > was added really long back -- git blame shows Linus' first git commit > introduced them in the git history, so it's pure legacy baggage. Two things here: - First you seem to have completely missed the fact that hvc_poll() can be called from interrupt time :-) Look at hvc_irq.c which is used by some backends. Maybe that can be "fixed" by deferring to a work queue, though it's nice to have the keyboard input have somewhat of a higher priority than anything else here. So unless that's fixed, or I missed something, that's a big NACK for now. - Then, are we certain that there's no case where the tty layer will call us with some lock held or in an atomic context ? To be honest, I've totally lost track of the locking rules in tty land lately so it might well be ok, but something to verify. Cheers, Ben.