| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-Id: <20250819010724.1338654-1-chenridong@huaweicloud.com>
Date: Tue, 19 Aug 2025 01:07:24 +0000
From: Chen Ridong <chenridong@...weicloud.com>
To: tj@...nel.org,
hannes@...xchg.org,
mkoutny@...e.com,
lizefan@...wei.com
Cc: cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org,
lujialin4@...wei.com,
chenridong@...wei.com,
hdanton@...a.com,
gaoyingjie@...ontech.com
Subject: [PATCH v6] cgroup: split cgroup_destroy_wq into 3 workqueues
From: Chen Ridong <chenridong@...wei.com>
A hung task can occur during [1] LTP cgroup testing when repeatedly
mounting/unmounting perf_event and net_prio controllers with
systemd.unified_cgroup_hierarchy=1. The hang manifests in
cgroup_lock_and_drain_offline() during root destruction.
Related case:
cgroup_fj_function_perf_event cgroup_fj_function.sh perf_event
cgroup_fj_function_net_prio cgroup_fj_function.sh net_prio
Call Trace:
cgroup_lock_and_drain_offline+0x14c/0x1e8
cgroup_destroy_root+0x3c/0x2c0
css_free_rwork_fn+0x248/0x338
process_one_work+0x16c/0x3b8
worker_thread+0x22c/0x3b0
kthread+0xec/0x100
ret_from_fork+0x10/0x20
Root Cause:
CPU0 CPU1
mount perf_event umount net_prio
cgroup1_get_tree cgroup_kill_sb
rebind_subsystems // root destruction enqueues
// cgroup_destroy_wq
// kill all perf_event css
// one perf_event css A is dying
// css A offline enqueues cgroup_destroy_wq
// root destruction will be executed first
css_free_rwork_fn
cgroup_destroy_root
cgroup_lock_and_drain_offline
// some perf descendants are dying
// cgroup_destroy_wq max_active = 1
// waiting for css A to die
Problem scenario:
1. CPU0 mounts perf_event (rebind_subsystems)
2. CPU1 unmounts net_prio (cgroup_kill_sb), queuing root destruction work
3. A dying perf_event CSS gets queued for offline after root destruction
4. Root destruction waits for offline completion, but offline work is
blocked behind root destruction in cgroup_destroy_wq (max_active=1)
Solution:
Split cgroup_destroy_wq into three dedicated workqueues:
cgroup_offline_wq – Handles CSS offline operations
cgroup_release_wq – Manages resource release
cgroup_free_wq – Performs final memory deallocation
This separation eliminates blocking in the CSS free path while waiting for
offline operations to complete.
[1] https://github.com/linux-test-project/ltp/blob/master/runtest/controllers
Fixes: 334c3679ec4b ("cgroup: reimplement rebind_subsystems() using cgroup_apply_control() and friends")
Reported-by: Gao Yingjie <gaoyingjie@...ontech.com>
Signed-off-by: Chen Ridong <chenridong@...wei.com>
Suggested-by: Teju Heo <tj@...nel.org>
---
kernel/cgroup/cgroup.c | 43 +++++++++++++++++++++++++++++++++++-------
1 file changed, 36 insertions(+), 7 deletions(-)
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 312c6a8b55bb..79b1d79f86a3 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -126,8 +126,31 @@ DEFINE_PERCPU_RWSEM(cgroup_threadgroup_rwsem);
* of concurrent destructions. Use a separate workqueue so that cgroup
* destruction work items don't end up filling up max_active of system_wq
* which may lead to deadlock.
+ *
+ * A cgroup destruction should enqueue work sequentially to:
+ * cgroup_offline_wq: use for css offline work
+ * cgroup_release_wq: use for css release work
+ * cgroup_free_wq: use for free work
+ *
+ * Rationale for using separate workqueues:
+ * The cgroup root free work may depend on completion of other css offline
+ * operations. If all tasks were enqueued to a single workqueue, this could
+ * create a deadlock scenario where:
+ * - Free work waits for other css offline work to complete.
+ * - But other css offline work is queued after free work in the same queue.
+ *
+ * Example deadlock scenario with single workqueue (cgroup_destroy_wq):
+ * 1. umount net_prio
+ * 2. net_prio root destruction enqueues work to cgroup_destroy_wq (CPUx)
+ * 3. perf_event CSS A offline enqueues work to same cgroup_destroy_wq (CPUx)
+ * 4. net_prio cgroup_destroy_root->cgroup_lock_and_drain_offline.
+ * 5. net_prio root destruction blocks waiting for perf_event CSS A offline,
+ * which can never complete as it's behind in the same queue and
+ * workqueue's max_active is 1.
*/
-static struct workqueue_struct *cgroup_destroy_wq;
+static struct workqueue_struct *cgroup_offline_wq;
+static struct workqueue_struct *cgroup_release_wq;
+static struct workqueue_struct *cgroup_free_wq;
/* generate an array of cgroup subsystem pointers */
#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
@@ -5558,7 +5581,7 @@ static void css_release_work_fn(struct work_struct *work)
cgroup_unlock();
INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
- queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
+ queue_rcu_work(cgroup_free_wq, &css->destroy_rwork);
}
static void css_release(struct percpu_ref *ref)
@@ -5567,7 +5590,7 @@ static void css_release(struct percpu_ref *ref)
container_of(ref, struct cgroup_subsys_state, refcnt);
INIT_WORK(&css->destroy_work, css_release_work_fn);
- queue_work(cgroup_destroy_wq, &css->destroy_work);
+ queue_work(cgroup_release_wq, &css->destroy_work);
}
static void init_and_link_css(struct cgroup_subsys_state *css,
@@ -5701,7 +5724,7 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
list_del_rcu(&css->sibling);
err_free_css:
INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
- queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
+ queue_rcu_work(cgroup_free_wq, &css->destroy_rwork);
return ERR_PTR(err);
}
@@ -5939,7 +5962,7 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
if (atomic_dec_and_test(&css->online_cnt)) {
INIT_WORK(&css->destroy_work, css_killed_work_fn);
- queue_work(cgroup_destroy_wq, &css->destroy_work);
+ queue_work(cgroup_offline_wq, &css->destroy_work);
}
}
@@ -6325,8 +6348,14 @@ static int __init cgroup_wq_init(void)
* We would prefer to do this in cgroup_init() above, but that
* is called before init_workqueues(): so leave this until after.
*/
- cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
- BUG_ON(!cgroup_destroy_wq);
+ cgroup_offline_wq = alloc_workqueue("cgroup_offline", 0, 1);
+ BUG_ON(!cgroup_offline_wq);
+
+ cgroup_release_wq = alloc_workqueue("cgroup_release", 0, 1);
+ BUG_ON(!cgroup_release_wq);
+
+ cgroup_free_wq = alloc_workqueue("cgroup_free", 0, 1);
+ BUG_ON(!cgroup_free_wq);
return 0;
}
core_initcall(cgroup_wq_init);
--
2.34.1
Powered by blists - more mailing lists