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cgroup: Annotate unlocked nr_populated_* accesses with READ_ONCE/WRITE_ONCE

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cgroup_update_populated() updates nr_populated_csets,
nr_populated_domain_children, and nr_populated_threaded_children under
css_set_lock, but cgroup_has_tasks(), cgroup_is_populated(), and
cgroup_can_be_thread_root() read them without holding it. Use
READ_ONCE/WRITE_ONCE.

Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2026-05-04 14:51:18 -10:00
parent 3360a5c16d
commit 44fabf0563
2 changed files with 24 additions and 8 deletions
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21
include/linux/cgroup.h
View File

@ -639,16 +639,29 @@ static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
return cgroup_is_descendant(cset->dfl_cgrp, ancestor);
}
/*
* Populated counters: writes happen under css_set_lock. The accessors below
* may read unlocked. What an unpopulated result means depends on context:
*
* - No lock held. Just a snapshot. May race with concurrent updates and is
* useful only as a hint.
*
* - cgroup_mutex held. Migration into the cgroup is blocked, so an observed
* !populated stays !populated until cgroup_mutex is dropped.
*
* - CSS_DYING set. The css can no longer be repopulated, so !populated is
* sticky once observed.
*/
static inline bool cgroup_has_tasks(struct cgroup *cgrp)
{
return cgrp->nr_populated_csets;
return READ_ONCE(cgrp->nr_populated_csets);
}
/* no synchronization, the result can only be used as a hint */
static inline bool cgroup_is_populated(struct cgroup *cgrp)
{
return cgrp->nr_populated_csets + cgrp->nr_populated_domain_children +
cgrp->nr_populated_threaded_children;
return READ_ONCE(cgrp->nr_populated_csets) +
READ_ONCE(cgrp->nr_populated_domain_children) +
READ_ONCE(cgrp->nr_populated_threaded_children);
}
/* returns ino associated with a cgroup */

11
kernel/cgroup/cgroup.c
View File

@ -404,7 +404,7 @@ static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
return false;
/* can only have either domain or threaded children */
if (cgrp->nr_populated_domain_children)
if (READ_ONCE(cgrp->nr_populated_domain_children))
return false;
/* and no domain controllers can be enabled */
@ -783,12 +783,15 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
bool was_populated = cgroup_is_populated(cgrp);
if (!child) {
cgrp->nr_populated_csets += adj;
WRITE_ONCE(cgrp->nr_populated_csets,
cgrp->nr_populated_csets + adj);
} else {
if (cgroup_is_threaded(child))
cgrp->nr_populated_threaded_children += adj;
WRITE_ONCE(cgrp->nr_populated_threaded_children,
cgrp->nr_populated_threaded_children + adj);
else
cgrp->nr_populated_domain_children += adj;
WRITE_ONCE(cgrp->nr_populated_domain_children,
cgrp->nr_populated_domain_children + adj);
}
if (was_populated == cgroup_is_populated(cgrp))