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

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

Ugh... please disregard this.  I hit send accidentally.

On 07/06/2012 01:14 PM, Corey Bryant wrote:
>
>
> 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
>
>

-- 
Regards,
Corey