netdev - Re: [PATCH v4 4/8] cgroup: Remove CGROUP_BUILTIN_SUBSYS

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Date:	Thu, 13 Sep 2012 08:57:39 +0200
From:	Daniel Wagner <wagi@...om.org>
To:	Li Zefan <lizefan@...wei.com>
CC:	netdev@...r.kernel.org, cgroups@...r.kernel.org,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	Gao feng <gaofeng@...fujitsu.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH v4 4/8] cgroup: Remove CGROUP_BUILTIN_SUBSYS_COUNT

Hi Li,

On 13.09.2012 08:41, Li Zefan wrote:
>> @@ -1321,11 +1321,13 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
>>   	 * take duplicate reference counts on a subsystem that's already used,
>>   	 * but rebind_subsystems handles this case.
>>   	 */
>> -	for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
>> +	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
>>   		unsigned long bit = 1UL << i;
>>
>>   		if (!(bit & opts->subsys_mask))
>>   			continue;
>> +		if (!subsys[i]->module)
>> +			continue;
>
> This check is not necessary. If it's builtin, try_module_get() will just return 1, and
> we're fine.

Yes, I didn't see the try_module_get. Although I think with leaving the 
test away it would change the behavior, e.g.

		if (!subsys[i]->module)
			continue;
		if (!try_module_get(subsys[i]->module)) {
			module_pin_failed = true;
			break;
		}

module_pin_failed would be set then and we would jump into the error 
code later.

This tests looks a bit ugly though I think leaving it away and relying 
on try_module_get() is not correct.

>> @@ -1437,6 +1443,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
>>   	INIT_LIST_HEAD(&cgrp->event_list);
>>   	spin_lock_init(&cgrp->event_list_lock);
>>   	simple_xattrs_init(&cgrp->xattrs);
>> +	memset(cgrp->subsys, 0, sizeof(cgrp->subsys));
>
> This seems an unrelated change, and is redundant. Am I missing something?

The reason why it is necessary to NULL all the entries in the array, is 
that task_cls_classid() and task_netprioidx() check the return pointer 
from task_subsys_state(). If it is NULL those function know that the 
subsystem is not ready to be used. Should I move this change to the next 
patch then?

cheers,
daniel
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