linux-kernel - Re: [PATCH v7 09/10] IPC: message queue copy feature introduced

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Message-ID: <CAKgNAkifPthwhBYzeXk24DOuJxV93_Uqr=XjAgPbdpLDbOjELA@mail.gmail.com>
Date:	Thu, 18 Oct 2012 12:39:09 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	akpm@...ux-foundation.org, catalin.marinas@....com,
	will.deacon@....com, dhowells@...hat.com, manfred@...orfullife.com,
	hughd@...gle.com, jmorris@...ei.org,
	kosaki.motohiro@...fujitsu.com, paulmck@...ux.vnet.ibm.com,
	sds@...ho.nsa.gov, devel@...nvz.org, a.p.zijlstra@...llo.nl,
	cmetcalf@...era.com, linux-driver@...gic.com,
	ron.mercer@...gic.com, viro@...iv.linux.org.uk,
	eparis@...isplace.org, tglx@...utronix.de,
	jitendra.kalsaria@...gic.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, ebiederm@...ssion.com,
	casey@...aufler-ca.com
Subject: Re: [PATCH v7 09/10] IPC: message queue copy feature introduced

On Thu, Oct 18, 2012 at 12:23 PM, Stanislav Kinsbursky
<skinsbursky@...allels.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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