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

[Oren Laadan]

Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl at librato.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.