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SubjectRe: [PATCH 2/2] cgroup: remove subsystem files when remounting cgroup
Hello Tejun

on 2012/12/01 03:27, Tejun Heo wrote:
> Hello, Gao.
>
> On Sat, Dec 01, 2012 at 12:21:29AM +0800, Gao feng wrote:
>> cgroup_clear_directroy is called by cgroup_d_remove_dir
>> and cgroup_remount.
>>
>> when we call cgroup_remount to remount the cgroup,the subsystem
>> may be unlinked from cgroupfs_root->subsys_list in rebind_subsystem,this
>> subsystem's files will not be removed in cgroup_clear_directroy.
>> And the system will panic when we try to access these files.
>>
>> this patch fix it.
>
> I'd really appreciated if you explain how it's fixed.
>
>> Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
>> ---
>> kernel/cgroup.c | 86 ++++++++++++++++++++++++++++++++++--------------------
>> 1 files changed, 54 insertions(+), 32 deletions(-)
>>
>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>> index e5233b7..90429d5 100644
>> --- a/kernel/cgroup.c
>> +++ b/kernel/cgroup.c
>> @@ -982,10 +982,14 @@ static void cgroup_clear_directory(struct dentry *dir, bool base_files,
>> unsigned long subsys_mask)
>> {
>> struct cgroup *cgrp = __d_cgrp(dir);
>> - struct cgroup_subsys *ss;
>> + int i;
>>
>> - for_each_subsys(cgrp->root, ss) {
>> + for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
>> struct cftype_set *set;
>> + struct cgroup_subsys *ss = subsys[i];
>> +
>> + if (ss == NULL)
>> + continue;
>
> Ummm... I can't say I like it. It tries to work around the problem
> that the backing subsys may go away earlier by changing how the
> subsystems are iterated so that the "gone" subsystems can be iterated,
> which really doesn't seem like a good diea. I mean, what if somebody
> tries to access the files inbetween? root_cgrp->subsys[] would be
> %NULL and segfault all the same.
>

Yes,You are right.I didn't consider this problem.

>> static void drop_parsed_module_refcounts(unsigned long subsys_mask)
>> @@ -1392,23 +1403,34 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
>> removed_mask = root->subsys_mask & ~opts.subsys_mask;
>>
>> /* Don't allow flags or name to change at remount */
>> + ret = -EINVAL;
>> if (opts.flags != root->flags ||
>> (opts.name && strcmp(opts.name, root->name))) {
>> - ret = -EINVAL;
>> drop_parsed_module_refcounts(opts.subsys_mask);
>> goto out_unlock;
>> }
>>
>> - ret = rebind_subsystems(root, opts.subsys_mask);
>> + /*
>> + * Add the reference of these removed subsystem,we
>> + * need use subsystems to remove their cgroup files.
>> + */
>> + ret = cgroup_get_module_refcounts(removed_mask);
>> if (ret) {
>> drop_parsed_module_refcounts(opts.subsys_mask);
>> goto out_unlock;
>> }
>>
>> + ret = rebind_subsystems(root, opts.subsys_mask);
>> + if (ret) {
>> + drop_parsed_module_refcounts(opts.subsys_mask | removed_mask);
>> + goto out_unlock;
>> + }
>> +
>> /* clear out any existing files and repopulate subsystem files */
>> cgroup_clear_directory(cgrp->dentry, false, removed_mask);
>> /* re-populate subsystem files */
>> cgroup_populate_dir(cgrp, false, added_mask);
>> + drop_parsed_module_refcounts(removed_mask);
>
> So, how about performing cgroup_clear_directory() *before*
> rebind_subsystems() and then restore it afterwards?
>

Get it,will fix this problem as you said.

Thanks!



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