[Devel] Re: [PATCH v3 08/10] IPC: message queue copy feature introduced

[Manfred Spraul]

Hi Stanislav,

2012/8/10 Stanislav Kinsbursky <skinsbursky at parallels.com>:
> 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. Also, copy counter was added to msg_queue structure. It's set to
> zero by default and increases by one on each copy operation and decreased by
> one on each receive operation until reaches zero.

Is msq->q_copy_cnt really necessary?
As far as I can see user space needs the ability to read the n-th message.

The implementation adds a state variable to the kernel, adds two
automatic updates of the state into msgrcv() (an increase during
MSG_COPY, a decrease during normal receives) and adds a msgctl() to
set the state to a certain value.

a) What about the simpler approach:
- if MSG_COPY is set, then @mtype is interpreted as the number of the
message that should be copied.
  If there are less than @mtype messages, then -ENOMSG is returned.

b) I do not understand the purpose of the decrease of msq->q_copy_cnt:
Do you want to handle normal msgrcv() calls in parallel with
msgrcv(|MSG_COPY) calls?
I don't think that this will work:
What if msq->q_copy_cnt is 1 and and msgrcv() call receives the 20th
message in the queue?

--
  Manfred