Messages in this thread | | | Date | Wed, 4 Jun 2014 16:40:28 +0800 | From | Li Zefan <> | Subject | Re: [PATCH 1/3] cgroup: don't destroy the default root |
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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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