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From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Date: Thu, 18 Oct 2012 12:39:09 +0200
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Subject: Re: [PATCH v7 09/10] IPC: message queue copy feature introduced
To: Stanislav Kinsbursky <skinsbursky@parallels.com>
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On Thu, Oct 18, 2012 at 12:23 PM, Stanislav Kinsbursky
<skinsbursky@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?)

Thanks,

Michael
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