linux-kernel - Re: [RFC v17][PATCH 22/60] c/r: external checkpoint of a task other than ourself

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Message-ID: <4A67E7D7.9060800@librato.com>
Date:	Thu, 23 Jul 2009 00:32:23 -0400
From:	Oren Laadan <orenl@...rato.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...l.org>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Dave Hansen <dave@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Oren Laadan <orenl@...columbia.edu>
Subject: Re: [RFC v17][PATCH 22/60] c/r: external checkpoint of a task	other
 than ourself



Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl@...rato.com):
>> Now we can do "external" checkpoint, i.e. act on another task.
> 
> ...
> 
>>  long do_checkpoint(struct ckpt_ctx *ctx, pid_t pid)
>>  {
>>  	long ret;
>>
>> +	ret = init_checkpoint_ctx(ctx, pid);
>> +	if (ret < 0)
>> +		return ret;
>> +
>> +	if (ctx->root_freezer) {
>> +		ret = cgroup_freezer_begin_checkpoint(ctx->root_freezer);
>> +		if (ret < 0)
>> +			return ret;
>> +	}
> 
> Self-checkpoint of a task in root freezer is now denied, though.
> 
> Was that intentional?

Yes.

"root freezer" is an arbitrary task in the checkpoint subtree or
container. It is used to verify that all checkpointed tasks - except
for current, if doing self-checkpoint - belong to the same freezer
group.

Since current is busy calling checkpoint(2), and since we only permit
checkpoint of (cgroup-) frozen tasks, then - by definition - it cannot
possibly belong to the same group. If it did, it would itself be frozen
like its fellows and unable to call checkpoint(2).

Oren.

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