From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:44667) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SoFvI-0002yp-Vd for qemu-devel@nongnu.org; Mon, 09 Jul 2012 11:31:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SoFvG-0007Il-Ti for qemu-devel@nongnu.org; Mon, 09 Jul 2012 11:30:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40362) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SoFvG-0007IS-Cv for qemu-devel@nongnu.org; Mon, 09 Jul 2012 11:30:50 -0400 Message-ID: <4FFAF91E.2010804@redhat.com> Date: Mon, 09 Jul 2012 17:30:38 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <1340390174-7493-1-git-send-email-coreyb@linux.vnet.ibm.com> <20120626091004.GA14451@redhat.com> <4FE9A0F0.2050809@redhat.com> <20120626175045.2c7011b3@doriath.home> <4FEA37A9.10707@linux.vnet.ibm.com> <4FEA3D9C.8080205@redhat.com> <4FF21A67.8010100@linux.vnet.ibm.com> <4FF31265.1000308@linux.vnet.ibm.com> <4FF316C9.5020100@redhat.com> <4FF31CFD.7030508@linux.vnet.ibm.com> <4FF325C8.4060401@redhat.com> <4FF3F806.7020307@redhat.com> <4FF5A331.9030108@linux.vnet.ibm.com> <4FF5A9D9.7080607@redhat.com> <4FF5C24E.9010008@linux.vnet.ibm.com> <4FF5C811.6040605@redhat.com> <4FF6ABD9.4040502@redhat.com> <4FF7232B.4040101@linux.vnet.ibm.com> <4FFAE4D7.5080700@redhat.com> <4FFAF78A.2040004@linux.vnet.ibm.com> In-Reply-To: <4FFAF78A.2040004@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Corey Bryant Cc: aliguori@us.ibm.com, stefanha@linux.vnet.ibm.com, libvir-list@redhat.com, qemu-devel@nongnu.org, Luiz Capitulino , pbonzini@redhat.com, Eric Blake Am 09.07.2012 17:23, schrieb Corey Bryant: >>> I think it would cause fds to sit on the monitor >>> until refcount gets to zero (monitor disconnects). Here's an example >>> without the in-use flag: >>> >>> 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with >>> refcount of 1 (incremented because of monitor reference); fd=4's remove >>> flag is initialized to off >>> 2. client calls 'device-add' with /dev/fdset/1 as the backing filename; >>> qemu_open() increments the refcount of fdset1 to 2 >>> 3. client crashes, so all fdsets are visited; fd=4 had not yet been >>> passed to 'remove-fd', so it's remove flag is off; refcount for fdset1 >>> is decremented to 1; fd=4 is left open because it is still in use by the >>> block device (refcount is 1) >>> 4. client re-establishes QMP connection, refcount for fdset1 is >>> incremented to 2; 'query-fds' lets client learn about fd=4 still being >>> open as part of fdset1 >>> 5. client calls 'remove-fd fdset=1 fd=4'; qemu turns on remove flag for >>> fd=4; but fd=4 remains open because refcount of fdset1 is 2 >> >> It also decreases the reference count because the monitor doesn't use it >> any more. > > I don't think that will work because refcount is for the entire fdset. > So we can't decrement the refcount for every fd that is removed from the > fdset. > > I think it is much simpler if we only increment refcount for an fdset on > qemu_open, and only decrement refcount on qemu_close. Ah right... So this would only work if we had explicit fdset-create/close commands, where the former would increase the refcount and the latter decrease it (fdset-open would be optional but I like symmetry) Maybe we need (or want) that anyway, but I need to think more about it first. >>> 6. qemu_close is called for fd=4; refcount for fdset1 is decremented to >>> 1; fd=4 remains open because monitor still references fdset1 (refcount >>> of fdset1 is 1) >> >> So here the refcount becomes 0 and the fdset is closed. >> > >>> 7. Sometime later.. QMP disconnects; refcount for fdset is decremented >>> to zero; fd=4 is closed >> >> The only question that is a bit unclear to me is whether a remove-fd on >> one monitor drops the refcount only for this monitor or for all >> monitors. However, both options can be implemented without additional >> flags or counters. > > Before we go back and forth on this thread, would you mind taking a look > at the last email I sent to Luiz? It includes all the design points > that I'm currently working from. I think it's a good level set and we > can work off that thread if there are still any issues. Ok, I'll have a look. Kevin