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cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock
The number of descendant cgroups and the number of dying descendant cgroups are currently synchronized using the cgroup_mutex. The number of descendant cgroups will be required by the cgroup v2 freezer, which will use it to determine if a cgroup is frozen (depending on total number of descendants and number of frozen descendants). It's not always acceptable to grab the cgroup_mutex, especially from quite hot paths (e.g. exit()). To avoid this, let's additionally synchronize these counters using the css_set_lock. So, it's safe to read these counters with either cgroup_mutex or css_set_lock locked, and for changing both locks should be acquired. Signed-off-by: Roman Gushchin <guro@fb.com> Signed-off-by: Tejun Heo <tj@kernel.org> Cc: kernel-team@fb.com
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@ -349,6 +349,11 @@ struct cgroup {
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* Dying cgroups are cgroups which were deleted by a user,
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* but are still existing because someone else is holding a reference.
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* max_descendants is a maximum allowed number of descent cgroups.
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*
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* nr_descendants and nr_dying_descendants are protected
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* by cgroup_mutex and css_set_lock. It's fine to read them holding
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* any of cgroup_mutex and css_set_lock; for writing both locks
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* should be held.
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*/
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int nr_descendants;
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int nr_dying_descendants;
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@ -4811,9 +4811,11 @@ static void css_release_work_fn(struct work_struct *work)
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if (cgroup_on_dfl(cgrp))
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cgroup_rstat_flush(cgrp);
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spin_lock_irq(&css_set_lock);
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for (tcgrp = cgroup_parent(cgrp); tcgrp;
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tcgrp = cgroup_parent(tcgrp))
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tcgrp->nr_dying_descendants--;
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spin_unlock_irq(&css_set_lock);
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cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
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cgrp->id = -1;
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@ -5031,12 +5033,14 @@ static struct cgroup *cgroup_create(struct cgroup *parent)
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if (ret)
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goto out_psi_free;
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spin_lock_irq(&css_set_lock);
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for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
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cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
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if (tcgrp != cgrp)
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tcgrp->nr_descendants++;
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}
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spin_unlock_irq(&css_set_lock);
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if (notify_on_release(parent))
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set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
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@ -5321,10 +5325,12 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
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if (parent && cgroup_is_threaded(cgrp))
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parent->nr_threaded_children--;
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spin_lock_irq(&css_set_lock);
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for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {
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tcgrp->nr_descendants--;
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tcgrp->nr_dying_descendants++;
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}
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spin_unlock_irq(&css_set_lock);
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cgroup1_check_for_release(parent);
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