On 2014/6/3 20:57, Tejun Heo wrote:
> Hello, Li.
>
> On Tue, Jun 03, 2014 at 12:04:38PM +0800, Li Zefan wrote:
>> static void cgroup_get(struct cgroup *cgrp)
>> {
>> WARN_ON_ONCE(cgroup_is_dead(cgrp));
>> - css_get(&cgrp->self);
>> + if (!(cgrp->self.flags & CSS_NO_REF))
>> + css_get(&cgrp->self);
>
> Hmmm? The same condition is tested by css_get(). Why should it be
> tested again here?
>
Oh, I completely ignored that.
>> static void cgroup_put(struct cgroup *cgrp)
>> {
>> - css_put(&cgrp->self);
>> + if (!(cgrp->self.flags & CSS_NO_REF))
>> + css_put(&cgrp->self);
>
> Ditto.
>
>> @@ -1781,10 +1783,12 @@ static void cgroup_kill_sb(struct super_block *sb)
>> * This prevents new mounts by disabling percpu_ref_tryget_live().
>> * cgroup_mount() may wait for @root's release.
>> */
>> - if (css_has_online_children(&root->cgrp.self))
>> + if (css_has_online_children(&root->cgrp.self)) {
>> cgroup_put(&root->cgrp);
>> - else
>> - percpu_ref_kill(&root->cgrp.self.refcnt);
>> + } else {
>> + if (root != &cgrp_dfl_root)
>> + percpu_ref_kill(&root->cgrp.self.refcnt);
>> + }
>
> As conceptually percpu_ref_kill() just puts the base ref and the
> dfl_root's refcnt never reaches zero, it won't actually trigger.
Yes it will, just try mount && umount.
I think it's because cgroup_get() is a no-op for CSS_NO_REF, so it has
only the base ref, so percpu_ref_iill() will actually schedule the
call to css_release().
> Hmmm.... wouldn't the above leak a ref each time the default hierarchy
> is unmounted tho? Shouldn't it be like the following?
>
cgroup_get() is a no-op for root cgroup of the default root, so there's
no leak, but still better to call cgroup_put().
I'll send an updated patch.
> if (root == &cgrp_dfl_root || css_has_online_children(...))
> cgroup_put(&root->cgrp);
> else
> percpu_ref_kill(...);
>
> Thanks.
>
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