From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MgWH8-0004wJ-RE for qemu-devel@nongnu.org; Thu, 27 Aug 2009 00:07:50 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MgWH4-0004sE-36 for qemu-devel@nongnu.org; Thu, 27 Aug 2009 00:07:50 -0400 Received: from [199.232.76.173] (port=47542 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MgWH3-0004sB-VY for qemu-devel@nongnu.org; Thu, 27 Aug 2009 00:07:46 -0400 Received: from gate.crashing.org ([63.228.1.57]:43894) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1MgWH3-0002Gj-Gd for qemu-devel@nongnu.org; Thu, 27 Aug 2009 00:07:45 -0400 From: Benjamin Herrenschmidt 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 Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: Extending virtio_console to support multiple ports List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Amit Shah Cc: kvm@vger.kernel.org, linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, miltonm@bga.com, qemu-devel@nongnu.org, borntraeger@de.ibm.com, brueckner@linux.vnet.ibm.com, virtualization@lists.linux-foundation.org, alan@linux.intel.com 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.