[Devel] Re: [PATCH v7 09/10] IPC: message queue copy feature introduced

[Stanislav Kinsbursky]

18.10.2012 14:39, Michael Kerrisk (man-pages) пишет:
> On Thu, Oct 18, 2012 at 12:23 PM, Stanislav Kinsbursky
> <skinsbursky at parallels.com> wrote:
>> This patch is required for checkpoint/restore in userspace.
>> IOW, c/r requires some way to get all pending IPC messages without deleting
>> them from the queue (checkpoint can fail and in this case tasks will be resumed,
>> so queue have to be valid).
>> To achive this, new operation flag MSG_COPY for sys_msgrcv() system call was
>> introduced. If this flag was specified, then mtype is interpreted as number of
>> the message to copy.
>> If MSG_COPY is set, then kernel will allocate dummy message with passed size,
>> and then use new copy_msg() helper function to copy desired message (instead of
>> unlinking it from the queue).
>>
>> Notes:
>> 1) Return -ENOSYS if MSG_COPY is specified, but CONFIG_CHECKPOINT_RESTORE is
>> not set.
>
> Stanislav,
>
> 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.

> Thanks,
>
> Michael
>


-- 
Best regards,
Stanislav Kinsbursky