| Re: [PATCH v7 09/10] IPC: message queue copy feature introduced [message #48490 is a reply to message #48489] |
Thu, 18 October 2012 12:05   |
Stanislav Kinsbursky
Messages: 683
Registered: October 2011
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Senior Member |
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18.10.2012 15:55, Michael Kerrisk (man-pages) пишет:
>>>>> A naive question, because I have not followed C/R closely. How do you
>>>>> deal with the case that other processes may be reading from the queue?
>>>>> (Or is that disabled during checkpointing?)
>>>>>
>>>>
>>>> To be honest, in this case behaviour in user-space is unpredictable.
>>>> I.e. if you have, for example, 5 messages in queue and going to peek them
>>>> all, and another process is reading the queue in the same time, then,
>>>> most
>>>> probably, you won't peek all the 5 and receive ENOMSG.
>>>> But this case can be easily handled by user-space application (number of
>>>> messages in queue can be discovered before peeking).
>>>>
>>>> Note, that in CRIU IPC resources will be collected when all processes to
>>>> migrate are frozen.
>>>
>>>
>>> Perhaps I am missing something fundamental, but how can C/R sanely do
>>> anything at all here?
>>>
>>> For example, suppose a process reads and processes a message after you
>>> read it with MSG_COPY. Then the remaining messages are all shifted by
>>> one position, and you are going to miss reading one of them. IIUC the
>>> idea of MSG_COPY is to allow you to retrieve a copy of all messages in
>>> the list. It sounds like there's no way this can be done reliably. So,
>>> what possible use does the operation have?
>>>
>>
>> First of all, this problem exist as is regardless to C/R feature or this
>> patch set. If you share some resource (like message queue in this particular
>> case) system-wide, then any process A can read out a message, which was send
>> by process B to process C. So, when processes uses IPC message queues, they
>> should be designed to handle such failures.
>>
>> Second, it's up to user-space how to handle such things. It's implied, that
>> user, trying to migrate some process, holding one end of queue, will also
>> migrate another process, holding second end.
>>
>> Third, there is IPC namespace, which isolates IPC objects. It can be used
>> for safe migration of process tree.
>
> Is there somewhere a *detailed* description of how this feature would
> be used? Lacking that, it's really hard to see how anything sane and
> reliable can be done with MSG_COPY.
>
These patches are used by CRIU already.
So, you can have a look at the CRIU source code:
http://git.criu.org/?p=crtools.git ;a=blob;f=ipc_ns.c;h=9e259fefcfc04ec0556bb722921545552e1c69f 3;hb=HEAD
Sanity and reliability on the level you are talking about can be achieved, only
if you'll freeze all message users before peeking.
--
Best regards,
Stanislav Kinsbursky
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