From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MgYps-0000fF-2v for qemu-devel@nongnu.org; Thu, 27 Aug 2009 02:51:52 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MgYpn-0000RU-Bm for qemu-devel@nongnu.org; Thu, 27 Aug 2009 02:51:51 -0400 Received: from [199.232.76.173] (port=46149 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MgYpm-0000RE-TJ for qemu-devel@nongnu.org; Thu, 27 Aug 2009 02:51:46 -0400 Received: from mx20.gnu.org ([199.232.41.8]:15181) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1MgYpm-0004RI-Hd for qemu-devel@nongnu.org; Thu, 27 Aug 2009 02:51:46 -0400 Received: from mx1.redhat.com ([209.132.183.28]) by mx20.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1MgYpl-0001iS-Lh for qemu-devel@nongnu.org; Thu, 27 Aug 2009 02:51:45 -0400 Date: Thu, 27 Aug 2009 12:21:02 +0530 From: Amit Shah Subject: Re: [Qemu-devel] Re: Extending virtio_console to support multiple ports Message-ID: <20090827065102.GA3218@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> <1251346023.20467.21.camel@pasglop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1251346023.20467.21.camel@pasglop> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Benjamin Herrenschmidt Cc: kvm@vger.kernel.org, borntraeger@de.ibm.com, qemu-devel@nongnu.org, miltonm@bga.com, linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org, brueckner@linux.vnet.ibm.com, virtualization@lists.linux-foundation.org, alan@linux.intel.com On (Thu) Aug 27 2009 [14:07:03], Benjamin Herrenschmidt wrote: > 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 Right! That's the obvious one. > 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. Hm, to maintain the current behaviour of poll() returning some poll_mask, the poll_mask could be made into an atomic variable with khvcd() updating it. But to have read at a higher priority than the other stuff, I don't quite see yet how that can be done. > 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 The other routines are open(), close(), write(), etc., and other kernel context (hvc_instantiate() and the khvcd thread). > totally lost track of the locking rules in tty land lately so it might > well be ok, but something to verify. Yes. Thanks for the response! Amit