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cgroup: Migrate tasks to the root css when a controller is rebound

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cgroup_apply_control_disable() defers kill_css_finish() while a css is
still populated, relying on css_update_populated() to fire the deferred
kill once the populated count reaches zero.

This deadlocks when a controller is rebound out of a hierarchy. Mounting
an implicit_on_dfl controller such as perf_event as a v1 hierarchy steals
it off the default hierarchy, and rebind_subsystems() kills its
per-cgroup csses while they are still populated. The migration run in the
same step keeps the old css for a controller no longer in the hierarchy's
mask, so no task is migrated off the dying csses. Their populated count
never reaches zero, the deferred kill_css_finish() never fires, and the
next cgroup_lock_and_drain_offline() hangs forever under cgroup_mutex.

That migration is already a no-op pass over the rebound subtree. Add
cgroup_rebind_ss_mask so find_existing_css_set() resolves the leaving
controllers to the root css. Their tasks are migrated there, the
per-cgroup csses depopulate, and cgroup_apply_control_disable() kills
them synchronously. The deferral stays correct for the rmdir and
controller-disable paths it was meant for.

Fixes: 1dffd95575 ("cgroup: Defer kill_css_finish() in cgroup_apply_control_disable()")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/41cd159c-54e5-45e0-81df-eaf36a6c028e@sirena.org.uk/
Reported-by: Bert Karwatzki <spasswolf@web.de>
Closes: https://lore.kernel.org/all/4e986b4ed7e16547805d54b6e67d09120bc4d2f2.camel@web.de/
Tested-by: Mark Brown <broonie@kernel.org>
Tested-by: Bert Karwatzki <spasswolf@web.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2026-06-01 08:56:04 -10:00
parent 6935f0496c
commit a99ce697ea
1 changed files with 31 additions and 4 deletions
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35
kernel/cgroup/cgroup.c
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@ -197,6 +197,14 @@ static u32 cgrp_dfl_implicit_ss_mask;
/* some controllers can be threaded on the default hierarchy */
static u32 cgrp_dfl_threaded_ss_mask;
/*
* Set across rebind_subsystems() to the controllers leaving a hierarchy.
* Guarded by cgroup_mutex. Makes find_existing_css_set() resolve them to the
* root css so the affected tasks are migrated there before
* cgroup_apply_control_disable() kills the per-cgroup csses.
*/
static u32 cgroup_rebind_ss_mask;
/* The list of hierarchy roots */
LIST_HEAD(cgroup_roots);
static int cgroup_root_count;
@ -1083,7 +1091,15 @@ static struct css_set *find_existing_css_set(struct css_set *old_cset,
* won't change, so no need for locking.
*/
for_each_subsys(ss, i) {
if (root->subsys_mask & (1UL << i)) {
if (unlikely(cgroup_rebind_ss_mask & (1UL << i))) {
/*
* @ss is leaving this hierarchy and its per-cgroup
* csses are about to be killed. Resolve to the
* surviving root css so the tasks are migrated there.
*/
template[i] = cgroup_css(&root->cgrp, ss);
WARN_ON_ONCE(!template[i]);
} else if (root->subsys_mask & (1UL << i)) {
/*
* @ss is in this hierarchy, so we want the
* effective css from @cgrp.
@ -1853,11 +1869,17 @@ int rebind_subsystems(struct cgroup_root *dst_root, u32 ss_mask)
struct cgroup *scgrp = &cgrp_dfl_root.cgrp;
/*
* Controllers from default hierarchy that need to be rebound
* are all disabled together in one go.
* Controllers leaving the default hierarchy are disabled
* together. cgroup_rebind_ss_mask makes cgroup_apply_control()
* migrate their tasks to the root css, so the per-cgroup csses
* are unpopulated when cgroup_finalize_control() kills them.
* Clear it before cgroup_finalize_control(), which does no
* css_set lookup.
*/
cgrp_dfl_root.subsys_mask &= ~dfl_disable_ss_mask;
cgroup_rebind_ss_mask = dfl_disable_ss_mask;
WARN_ON(cgroup_apply_control(scgrp));
cgroup_rebind_ss_mask = 0;
cgroup_finalize_control(scgrp, 0);
}
@ -1871,9 +1893,14 @@ int rebind_subsystems(struct cgroup_root *dst_root, u32 ss_mask)
WARN_ON(!css || cgroup_css(dcgrp, ss));
if (src_root != &cgrp_dfl_root) {
/* disable from the source */
/*
* Disable from the source, migrating its tasks to the
* root css first (see cgroup_rebind_ss_mask).
*/
src_root->subsys_mask &= ~(1 << ssid);
cgroup_rebind_ss_mask = 1 << ssid;
WARN_ON(cgroup_apply_control(scgrp));
cgroup_rebind_ss_mask = 0;
cgroup_finalize_control(scgrp, 0);
}