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

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

On 07/09/2012 01:48 PM, Luiz Capitulino wrote:
> On Mon, 09 Jul 2012 13:35:19 -0400
> Corey Bryant <coreyb@linux.vnet.ibm.com> wrote:
>
>>
>>
>> On 07/09/2012 11:46 AM, Kevin Wolf wrote:
>>> Am 09.07.2012 17:05, schrieb Corey Bryant:
>>>> I'm not sure this is an issue with current design.  I know things have
>>>> changed a bit as the email threads evolved, so I'll paste the current
>>>> design that I am working from.  Please let me know if you still see any
>>>> issues.
>>>>
>>>> FD passing:
>>>> -----------
>>>> New monitor commands enable adding/removing an fd to/from a set.  New
>>>> monitor command query-fdsets enables querying of current monitor fdsets.
>>>>     The set of fds should all refer to the same file, with each fd having
>>>> different access flags (ie. O_RDWR, O_RDONLY).  qemu_open can then dup
>>>> the fd that has the matching access mode flags.
>>>>
>>>> Design points:
>>>> --------------
>>>> 1. add-fd
>>>> -> fd is passed via SCM rights and qemu adds fd to first unused fdset
>>>> (e.g. /dev/fdset/1)
>>>> -> add-fd monitor function initializes the monitor inuse flag for the
>>>> fdset to true
>>>> -> add-fd monitor function initializes the remove flag for the fd to false
>>>> -> add-fd returns fdset number and received fd number (e.g fd=3) to caller
>>>>
>>>> 2. drive_add file=/dev/fdset/1
>>>> -> qemu_open uses the first fd in fdset1 that has access flags matching
>>>> the qemu_open action flags and has remove flag set to false
>>>> -> qemu_open increments refcount for the fdset
>>>> -> Need to make sure that if a command like 'device-add' fails that
>>>> refcount is not incremented
>>>>
>>>> 3. add-fd fdset=1
>>>> -> fd is passed via SCM rights
>>>> -> add-fd monitor function adds the received fd to the specified fdset
>>>> (or fails if fdset doesn't exist)
>>>> -> add-fd monitor function initializes the remove flag for the fd to false
>>>> -> add-fd returns fdset number and received fd number (e.g fd=4) to caller
>>>>
>>>> 4. block-commit
>>>> -> qemu_open performs "reopen" by using the first fd from the fdset that
>>>> has access flags matching the qemu_open action flags and has remove flag
>>>> set to false
>>>> -> qemu_open increments refcount for the fdset
>>>> -> Need to make sure that if a command like 'block-commit' fails that
>>>> refcount is not incremented
>>>>
>>>> 5. remove-fd fdset=1 fd=4
>>>> -> remove-fd monitor function fails if fdset doesn't exist
>>>> -> remove-fd monitor function turns on remove flag for fd=4
>>>
>>> What was again the reason why we keep removed fds in the fdset at all?
>>
>> Because if refcount is > 0 for the fd set, then the fd could be in use
>> by a block device.  So we keep it around until refcount is decremented
>> to zero, at which point it is safe to close.
>
> But then the refcount is associated with the set, not with any particular fd.
>

Exactly, yes that's what we're doing.  Sorry, I thought that was clear 
in the design overview I sent earlier today.

-- 
Regards,
Corey