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cgroup: Update comments about task exit path

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We no longer take cgroup_mutex in cgroup_exit and the exiting tasks are
not moved to init_css_set, reflect that in several comments to prevent
confusion.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Michal Koutný 2019-10-04 12:57:39 +02:00 committed by Tejun Heo
parent 61e867fde2
commit e7c7b1d85d
1 changed files with 9 additions and 20 deletions
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29
kernel/cgroup/cgroup.c
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@ -899,8 +899,7 @@ static void css_set_move_task(struct task_struct *task,
/*
* We are synchronized through cgroup_threadgroup_rwsem
* against PF_EXITING setting such that we can't race
* against cgroup_exit() changing the css_set to
* init_css_set and dropping the old one.
* against cgroup_exit()/cgroup_free() dropping the css_set.
*/
WARN_ON_ONCE(task->flags & PF_EXITING);
@ -1432,9 +1431,8 @@ struct cgroup *task_cgroup_from_root(struct task_struct *task,
struct cgroup_root *root)
{
/*
* No need to lock the task - since we hold cgroup_mutex the
* task can't change groups, so the only thing that can happen
* is that it exits and its css is set back to init_css_set.
* No need to lock the task - since we hold css_set_lock the
* task can't change groups.
*/
return cset_cgroup_from_root(task_css_set(task), root);
}
@ -6030,7 +6028,7 @@ void cgroup_post_fork(struct task_struct *child)
struct css_set *cset;
spin_lock_irq(&css_set_lock);
cset = task_css_set(current);
cset = task_css_set(current); /* current is @child's parent */
if (list_empty(&child->cg_list)) {
get_css_set(cset);
cset->nr_tasks++;
@ -6073,20 +6071,8 @@ void cgroup_post_fork(struct task_struct *child)
* cgroup_exit - detach cgroup from exiting task
* @tsk: pointer to task_struct of exiting process
*
* Description: Detach cgroup from @tsk and release it.
* Description: Detach cgroup from @tsk.
*
* Note that cgroups marked notify_on_release force every task in
* them to take the global cgroup_mutex mutex when exiting.
* This could impact scaling on very large systems. Be reluctant to
* use notify_on_release cgroups where very high task exit scaling
* is required on large systems.
*
* We set the exiting tasks cgroup to the root cgroup (top_cgroup). We
* call cgroup_exit() while the task is still competent to handle
* notify_on_release(), then leave the task attached to the root cgroup in
* each hierarchy for the remainder of its exit. No need to bother with
* init_css_set refcnting. init_css_set never goes away and we can't race
* with migration path - PF_EXITING is visible to migration path.
*/
void cgroup_exit(struct task_struct *tsk)
{
@ -6096,7 +6082,8 @@ void cgroup_exit(struct task_struct *tsk)
/*
* Unlink from @tsk from its css_set. As migration path can't race
* with us, we can check css_set and cg_list without synchronization.
* with us (thanks to cgroup_threadgroup_rwsem), we can check css_set
* and cg_list without synchronization.
*/
cset = task_css_set(tsk);
@ -6112,6 +6099,8 @@ void cgroup_exit(struct task_struct *tsk)
spin_unlock_irq(&css_set_lock);
} else {
/* Take reference to avoid freeing init_css_set in cgroup_free,
* see cgroup_fork(). */
get_css_set(cset);
}