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

On 07/05/2012 10:51 AM, Kevin Wolf wrote: Heh :) Good point. I thought it would be useful to run through the examples again with each fdset having a refcount, and each fd having an in-use flag. One difference though is that refcount is only incremented/decremented when qemu_open/qemu_close are called. I think this works and covers Kevin's concerns. Actually it might be a bit simpler too. 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 crashes, so all tracked fds are visited; fd=4 had not yet been passed to 'remove-fd', so it's in-use flag is on; in-use flag is turned off and fd=4 is left open because it is still in use by the block device (refcount is 1) 4. client re-establishes QMP connection, so all tracked fds are visited, and in-use flags are turned back on; 'query-fds' lets client learn about fd=4 still being open as part of 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 '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 -- Regards, Corey