linux-kernel - Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change an ID

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Date:	Wed, 30 Jan 2008 00:06:56 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	pierre.peiffer@...l.net
Cc:	linux-kernel@...r.kernel.org, containers@...ts.linux-foundation.org
Subject: Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change an ID

On Tue, Jan 29, 2008 at 05:02:38PM +0100, pierre.peiffer@...l.net wrote:
> This patch provides three new API to change the ID of an existing
> System V IPCs.
> 
> These APIs are:
> 	long msg_chid(struct ipc_namespace *ns, int id, int newid);
> 	long sem_chid(struct ipc_namespace *ns, int id, int newid);
> 	long shm_chid(struct ipc_namespace *ns, int id, int newid);
> 
> They return 0 or an error code in case of failure.
> 
> They may be useful for setting a specific ID for an IPC when preparing
> a restart operation.
> 
> To be successful, the following rules must be respected:
> - the IPC exists (of course...)
> - the new ID must satisfy the ID computation rule.
> - the entry in the idr corresponding to the new ID must be free.

>  ipc/util.c          |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  ipc/util.h          |    1 +
>  8 files changed, 197 insertions(+)

For the record, OpenVZ uses "create with predefined ID" method which
leads to less code. For example, change at the end is all we want from
ipc/util.c .

Also, if ids were A and B at the moment of checkpoint, and during
restart they became B and A you'll get collision in both ways which you
techically can avoid by classic "tmp = A, A = B, B = tmp" but you also
can avoid all other loops just by creating with ID you need.
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