linux-kernel - Re: [PATCH v3 08/10] IPC: message queue copy feature introduced

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Date:	Sat, 11 Aug 2012 13:20:58 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	akpm@...ux-foundation.org, eparis@...isplace.org,
	a.p.zijlstra@...llo.nl, arnd@...db.de, hughd@...gle.com,
	linux-kernel@...r.kernel.org, cmetcalf@...era.com,
	yeohc@....ibm.com, linux-security-module@...r.kernel.org,
	viro@...iv.linux.org.uk, kosaki.motohiro@...fujitsu.com,
	hpa@...or.com, casey@...aufler-ca.com,
	sds@...ho.nsa.govjames.l.morris, devel@...nvz.org
Subject: Re: [PATCH v3 08/10] IPC: message queue copy feature introduced

Hi Stanislav,

2012/8/10 Stanislav Kinsbursky <skinsbursky@...allels.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
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