Re: [libvirt] [Qemu-devel] [PATCH v4 0/7] file descriptor passing using pass-fd

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 > I think we need the granularity of having an in-use flag per fd. Here's an example of why: 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with refcount of 1; 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. qemu_close is called for fd=4; refcount for fdset1 is decremented to 1; fd=4 remains open as part of fdset1 6. QMP disconnects; refcount for fdset is decremented to zero; fd=4 is closed and fdset1 is freed 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with refcount of 1 (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, but the command fails for some other reason, so the refcount is still 1 at the end of the command (although it may have been temporarily incremented then decremented during the command) 3. client calls 'remove-fd fdset=1 fd=4' to deal with the failure (or QMP connection is closed), so qemu turns on the remove flag for fd=4; refcount is 0 so all fds in fdset1 are closed I think we need the granularity of having an in-use flag per fd. If we track monitor in-use in the reference count, then we are tracking it for the fdset and I think this could cause leaks. In the following example, we have a refcount for the fdset, an in-use flag per fd, and a remove flag per fd. We're only incrementing/decrementing refcount in qemu_open/qemu_close. In the following example, we have a refcount for the fdset, and a remove flag per fd. We're incrementing refcount when the fdset is first created, when QMP re-connects, and in qemu_open. We're decrementing refcount when QMP disconnects, and in qemu_close. 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with refcount of 1; fd=4's remove flag is initialized to off 2. client crashes, so all tracked fdsets are visited; fd=4 has not yet been passed to 'remove-fd', so its remove flag is off; in-use flags are turned off and both fds are left open because the set is still in use by the block device (refcount is 1) 1. client calls 'add-fd', qemu is now tracking fd=4 in fdset1 with refcount of 1; 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. qemu_close is called for fd=4; refcount for fdset1 is decremented to 1; fd=4 remains open as part of fdset1 6. QMP disconnects; refcount for fdset is decremented to zero; fd=4 is closed and fdset1 is freed 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 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, but the command fails for some other reason, so the refcount is still 0 at the end of the command (although it may have been temporarily incremented then decremented during the command) 3. client calls 'remove-fd fdset=1 fd=4' to deal with the failure (or QMP connection is closed), so qemu marks fd=4 as no longer in-use by the monitor; refcount is 0 so all fds in fdset1 are closed 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 'add-fd', qemu is now tracking fd=5 in fdset1 with refcount still 0; fd=5's in-use flag is turned on 3. client crashes, so all tracked fds are visited; fd=4 and fd=5 had not yet been passed to 'remove-fd', so their in-use flags are on; in-use flags are turned off; refcount of fdset1 is 0 so qemu closes all fds in fdset1 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 'add-fd', qemu is now tracking fd=5 in fdset1 with refcount still 0; fd=5's in-use flag is turned on 3. client calls 'remove-fd fdset=1 fd=4', so qemu marks fd=4 as no longer in-use by the monitor, and fd=4 is closed since the refcount is 0; fd=5 remains open 1. client calls 'add-fd', qemu is now tracking fd=4 (O_RDWR) in fdset1 with refcount of 0; fd=4's in-use flag is turned on 2. client calls 'add-fd', qemu is now tracking fd=5 (O_RDONLY) in fdset1 with refcount still 0; fd=5's in-use flag is turned on 3. client calls 'device-add' with /dev/fdset/1 as the backing filename and r/w, so qemu_open() dup()s fd 4 and increments the refcount to 1 4. client calls 'remove-fd fdset=1 fd=4', so qemu marks fd=4 as no longer in-use by the monitor, but fd=4 remains open because refcount of fdset1 is 1 5. client calls 'remove-fd fdset=1 fd=5', so qemu marks fd=5 as no longer in-use by the monitor, and fd=5 remains open because refcount of fdset1 is 1 6. qemu_close(fd=4) is called, refcount of fdset1 is decremented; both fds are closed since refcount is 0 and their in-use flags are off 1. client calls 'add-fd', qemu is now tracking fd=4 (O_RDWR) in fdset1 with refcount of 0; fd=4's in-use flag is turned on 2. client calls 'add-fd', qemu is now tracking fd=5 (O_RDONLY) in fdset1 with refcount still 0; fd=5's in-use flag is turned on 3. client calls 'device-add' with /dev/fdset/1 as the backing filename and r/w, so qemu_open() dup()s fd 4 and increments the refcount to 1 4. client crashes, so all tracked fds are visited; fd=4 and fd=5 have not yet been passed to 'remove-fd', so their in-use flags are on; in-use flags are turned off and both fds are left open because the set is still in use by the block device (refcount is 1) 5. qemu_close(fd=4) is called, refcount of fdset1 is decremented; both fds are closed since refcount is 0 and their in-use flags are off