From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:44645) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SnCXA-0002li-Ql for qemu-devel@nongnu.org; Fri, 06 Jul 2012 13:41:38 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SnCX8-0003ol-CA for qemu-devel@nongnu.org; Fri, 06 Jul 2012 13:41:36 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:43627) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SnCX8-0003nl-6s for qemu-devel@nongnu.org; Fri, 06 Jul 2012 13:41:34 -0400 Received: from /spool/local by e2.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 6 Jul 2012 13:41:21 -0400 Received: from d01relay03.pok.ibm.com (d01relay03.pok.ibm.com [9.56.227.235]) by d01dlp01.pok.ibm.com (Postfix) with ESMTP id 97B9D38C803A for ; Fri, 6 Jul 2012 13:41:01 -0400 (EDT) Received: from d01av03.pok.ibm.com (d01av03.pok.ibm.com [9.56.224.217]) by d01relay03.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id q66Hf1IN368500 for ; Fri, 6 Jul 2012 13:41:01 -0400 Received: from d01av03.pok.ibm.com (loopback [127.0.0.1]) by d01av03.pok.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id q66Hf1rH013461 for ; Fri, 6 Jul 2012 14:41:01 -0300 Message-ID: <4FF7232B.4040101@linux.vnet.ibm.com> Date: Fri, 06 Jul 2012 13:40:59 -0400 From: Corey Bryant 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> In-Reply-To: <4FF6ABD9.4040502@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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: Kevin Wolf 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 On 07/06/2012 05:11 AM, Kevin Wolf wrote: > Am 05.07.2012 19:00, schrieb Eric Blake: >> On 07/05/2012 10:35 AM, Corey Bryant wrote: >>> 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with >>> refcount of 0; fd=4's in-use flag is turned on >>> 2. client calls 'device-add' with /dev/fdset/1 as the backing filename, >>> so qemu_open() increments the refcount of fdset1 to 1 >>> 3. client calls 'remove-fd fdset=1 fd=4', so qemu marks fd=4 as no >>> longer in-use by the monitor, and is left open because it is in use by >>> the block device (refcount is 1) >>> 4. client crashes, so all tracked fds are visited; fd=4 is not in-use >>> but refcount is 1 so it is not closed >> 5. client re-establishes QMP connection, so all tracked fds are visited. >> What happens to the fd=4 in-use flag? >> >> ...but what are the semantics here? >> >> If we _always_ turn the in-use flag back on, then that says that even >> though libvirt successfully asked to close the fd, the reconnection >> means that libvirt once again has to ask to close things. >> >> If we _never_ turn the in-use flag back on, then we've broken the first >> case above where we want an in-use fd to come back into use after a crash. >> >> Maybe that argues for two flags per fd: an in-use flag (there is >> currently a monitor connection that can manipulate the fd, either >> because it passed in the fd or because it reconnected) and a removed >> flag (a monitor called remove-fd, and no longer wants to know about the >> fd, even if it crashes and reconnects). > > I was in fact just going to suggest a removed flag as well, however > combined with including the monitor connections in the refcount instead > of an additional flag. This would also allow to have (the currently > mostly hypothetical case of) multiple QMP monitors without interesting > things happening. > > Maybe I'm missing some point that the inuse flag would allow and > including monitors in the refcount doesn't. Is there one? > > Kevin > Ok let me try this again. I was going through some of the examples and I think we need a separate in-use flag. Otherwise, when refcount gets to 1, we don't know if it is because of a monitor reference or a block device reference. 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 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) 7. Sometime later.. QMP disconnects; refcount for fdset is decremented to zero; fd=4 is closed In the following case, we have an in-use and remove flag per fd and we only increment/decrement refcount on qemu_open/qemu_close: 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with refcount of 0; fd=4's remove flag is initialized to off and in-use flag is initialized to on 2. client calls 'device-add' with /dev/fdset/1 as the backing filename; qemu_open() increments the refcount of fdset1 to 1 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; fd=4's in-use flag is turned off; fd=4 is left open because it is still in-use by the block device (refcount is still 1) 4. client re-establishes QMP connection, refcount for fdset1 is still 1; fd=4's in-use flag is turned on; '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 1 6. qemu_close is called for fd=4; refcount for fdset1 is decremented to 0; fd=4 is closed because (refcount == 0 && (!inuse || removed)) is true 7. Sometime later.. QMP disconnects -- Regards, Corey