[PATCH] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[PATCH] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Junnan Zhang]

From: Junnan Zhang <zhangjn11@chinatelecom.cn>

cgroup1_pidlist_destroy_all() flushes the global
cgroup_pidlist_destroy_wq while destroying a cgroup. Because all cgroup
v1 pidlist destruction work items are queued on the same shared workqueue,
a single slow or stuck work item (e.g. waiting for pidlist_mutex held by a
user-space reader) blocks every concurrent cgroup destruction path.

This can lead to kworker tasks stuck in flush_workqueue() for over
hung_task_timeout seconds, as observed on busy systems running Docker or
Kubernetes workloads.

  INFO: task kworker/0:1:1438499 blocked for more than 120 seconds.
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  kworker/0:1 D 0 1438499 2 0x80000080
  Workqueue: cgroup_destroy css_free_rwork_fn
  ? __schedule+0x296/0x900
  schedule+0x28/0x80
  schedule_timeout+0x1ee/0x3a0
  ? kvm_sched_clock_read+0xd/0x20
  wait_for_completion+0x12c/0x190
  ? wake_up_q+0x70/0x70
  flush_workqueue+0x132/0x430
  ? cgroup1_pidlist_destroy_all+0x7c/0xa0
  cgroup1_pidlist_destroy_all+0x7c/0xa0
  css_free_rwork_fn+0xb5/0x390
  process_one_work+0x195/0x3e0
  worker_thread+0x30/0x390
  ? process_one_work+0x3e0/0x3e0
  kthread+0x113/0x130
  ? kthread_create_worker_on_cpu+0x70/0x70
  ret_from_fork+0x1f/0x40

Fix it by moving the cgroup's pidlists to a local orphan list under
pidlist_mutex, clearing their ->owner pointer, and then cancelling each
pidlist's delayed work outside the lock. The destroy work function now
checks ->owner and skips freeing orphaned pidlists, so
cgroup1_pidlist_destroy_all() can free them safely without flushing the
whole shared workqueue.

Signed-off-by: Junnan Zhang <zhangjn11@chinatelecom.cn>
Signed-off-by: Shouxin Sun <sunshx@chinatelecom.cn>
---
 kernel/cgroup/cgroup-v1.c | 53 ++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 15 deletions(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index a4337c9b5287..874fe4dc6ffd 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -206,13 +206,31 @@ struct cgroup_pidlist {
 void cgroup1_pidlist_destroy_all(struct cgroup *cgrp)
 {
 	struct cgroup_pidlist *l, *tmp_l;
+	LIST_HEAD(orphan);
+
+	/*
+	 * Move pidlists to a local orphan list and mark them as owner-less.
+	 * The destroy work function will see ->owner == NULL and skip freeing.
+	 * We then cancel and free them outside pidlist_mutex to avoid
+	 * flush_workqueue() blocking on the shared workqueue.
+	 */
 
 	mutex_lock(&cgrp->pidlist_mutex);
-	list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links)
-		mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork, 0);
+	list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links) {
+		list_del(&l->links);
+		l->owner = NULL;
+		list_add(&l->links, &orphan);
+	}
 	mutex_unlock(&cgrp->pidlist_mutex);
 
-	flush_workqueue(cgroup_pidlist_destroy_wq);
+	list_for_each_entry_safe(l, tmp_l, &orphan, links) {
+		list_del(&l->links);
+		cancel_delayed_work_sync(&l->destroy_dwork);
+		kvfree(l->list);
+		put_pid_ns(l->key.ns);
+		kfree(l);
+	}
+
 	BUG_ON(!list_empty(&cgrp->pidlists));
 }
 
@@ -222,21 +240,26 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
 	struct cgroup_pidlist *l = container_of(dwork, struct cgroup_pidlist,
 						destroy_dwork);
 	struct cgroup_pidlist *tofree = NULL;
+	struct cgroup *owner;
 
-	mutex_lock(&l->owner->pidlist_mutex);
+	owner = l->owner;
+	if (owner) {
+		mutex_lock(&owner->pidlist_mutex);
 
-	/*
-	 * Destroy iff we didn't get queued again.  The state won't change
-	 * as destroy_dwork can only be queued while locked.
-	 */
-	if (!delayed_work_pending(dwork)) {
-		list_del(&l->links);
-		kvfree(l->list);
-		put_pid_ns(l->key.ns);
-		tofree = l;
-	}
+		/*
+		 * Destroy iff we didn't get queued again and we're still
+		 * owned by the cgroup.  If ->owner was cleared by
+		 * cgroup1_pidlist_destroy_all(), it will free us.
+		 */
+		if (l->owner == owner && !delayed_work_pending(dwork)) {
+			list_del(&l->links);
+			kvfree(l->list);
+			put_pid_ns(l->key.ns);
+			tofree = l;
+		}
 
-	mutex_unlock(&l->owner->pidlist_mutex);
+		mutex_unlock(&l->owner->pidlist_mutex);
+	}
 	kfree(tofree);
 }
 
-- 
2.43.0

[PATCH v2] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Junnan Zhang]

From: Junnan Zhang <zhangjn11@chinatelecom.cn>

cgroup1_pidlist_destroy_all() flushes the global
cgroup_pidlist_destroy_wq while destroying a cgroup. Because all cgroup
v1 pidlist destruction work items are queued on the same shared workqueue,
a single slow or stuck work item (e.g. waiting for pidlist_mutex held by a
user-space reader) blocks every concurrent cgroup destruction path.

This can lead to kworker tasks stuck in flush_workqueue() for over
hung_task_timeout seconds, as observed on busy systems running Docker or
Kubernetes workloads.

  INFO: task kworker/0:1:1438499 blocked for more than 120 seconds.
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  kworker/0:1 D 0 1438499 2 0x80000080
  Workqueue: cgroup_destroy css_free_rwork_fn
  ? __schedule+0x296/0x900
  schedule+0x28/0x80
  schedule_timeout+0x1ee/0x3a0
  ? kvm_sched_clock_read+0xd/0x20
  wait_for_completion+0x12c/0x190
  ? wake_up_q+0x70/0x70
  flush_workqueue+0x132/0x430
  ? cgroup1_pidlist_destroy_all+0x7c/0xa0
  cgroup1_pidlist_destroy_all+0x7c/0xa0
  css_free_rwork_fn+0xb5/0x390
  process_one_work+0x195/0x3e0
  worker_thread+0x30/0x390
  ? process_one_work+0x3e0/0x3e0
  kthread+0x113/0x130
  ? kthread_create_worker_on_cpu+0x70/0x70
  ret_from_fork+0x1f/0x40

Fix it by moving the cgroup's pidlists to a local orphan list under
pidlist_mutex, clearing their ->owner pointer, and then cancelling each
pidlist's delayed work outside the lock. The destroy work function now
checks ->owner and skips freeing orphaned pidlists, so
cgroup1_pidlist_destroy_all() can free them safely without flushing the
whole shared workqueue.

Signed-off-by: Junnan Zhang <zhangjn11@chinatelecom.cn>
Signed-off-by: Shouxin Sun <sunshx@chinatelecom.cn>

---
v1 -> v2:
 - Fix a NULL pointer dereference in cgroup_pidlist_destroy_work_fn():
   use the cached owner pointer for mutex_unlock(), as ->owner may have
   been cleared concurrently by cgroup1_pidlist_destroy_all(). Spotted
   by Sashiko AI review.
---
 kernel/cgroup/cgroup-v1.c | 53 ++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 15 deletions(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index a4337c9b5287..88b10cd59c7b 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -206,13 +206,31 @@ struct cgroup_pidlist {
 void cgroup1_pidlist_destroy_all(struct cgroup *cgrp)
 {
 	struct cgroup_pidlist *l, *tmp_l;
+	LIST_HEAD(orphan);
+
+	/*
+	 * Move pidlists to a local orphan list and mark them as owner-less.
+	 * The destroy work function will see ->owner == NULL and skip freeing.
+	 * We then cancel and free them outside pidlist_mutex to avoid
+	 * flush_workqueue() blocking on the shared workqueue.
+	 */
 
 	mutex_lock(&cgrp->pidlist_mutex);
-	list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links)
-		mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork, 0);
+	list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links) {
+		list_del(&l->links);
+		l->owner = NULL;
+		list_add(&l->links, &orphan);
+	}
 	mutex_unlock(&cgrp->pidlist_mutex);
 
-	flush_workqueue(cgroup_pidlist_destroy_wq);
+	list_for_each_entry_safe(l, tmp_l, &orphan, links) {
+		list_del(&l->links);
+		cancel_delayed_work_sync(&l->destroy_dwork);
+		kvfree(l->list);
+		put_pid_ns(l->key.ns);
+		kfree(l);
+	}
+
 	BUG_ON(!list_empty(&cgrp->pidlists));
 }
 
@@ -222,21 +240,26 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
 	struct cgroup_pidlist *l = container_of(dwork, struct cgroup_pidlist,
 						destroy_dwork);
 	struct cgroup_pidlist *tofree = NULL;
+	struct cgroup *owner;
 
-	mutex_lock(&l->owner->pidlist_mutex);
+	owner = l->owner;
+	if (owner) {
+		mutex_lock(&owner->pidlist_mutex);
 
-	/*
-	 * Destroy iff we didn't get queued again.  The state won't change
-	 * as destroy_dwork can only be queued while locked.
-	 */
-	if (!delayed_work_pending(dwork)) {
-		list_del(&l->links);
-		kvfree(l->list);
-		put_pid_ns(l->key.ns);
-		tofree = l;
-	}
+		/*
+		 * Destroy iff we didn't get queued again and we're still
+		 * owned by the cgroup.  If ->owner was cleared by
+		 * cgroup1_pidlist_destroy_all(), it will free us.
+		 */
+		if (l->owner == owner && !delayed_work_pending(dwork)) {
+			list_del(&l->links);
+			kvfree(l->list);
+			put_pid_ns(l->key.ns);
+			tofree = l;
+		}
 
-	mutex_unlock(&l->owner->pidlist_mutex);
+		mutex_unlock(&owner->pidlist_mutex);
+	}
 	kfree(tofree);
 }
 
-- 
2.43.0

[PATCH v3] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Junnan Zhang]

From: Junnan Zhang <zhangjn11@chinatelecom.cn>

cgroup1_pidlist_destroy_all() flushes the global
cgroup_pidlist_destroy_wq while destroying a cgroup. Because all cgroup
v1 pidlist destruction work items are queued on the same shared workqueue,
a single slow or stuck work item (e.g. waiting for pidlist_mutex held by a
user-space reader) blocks every concurrent cgroup destruction path.

This can lead to kworker tasks stuck in flush_workqueue() for over
hung_task_timeout seconds, as observed on busy systems running Docker or
Kubernetes workloads.

  INFO: task kworker/0:1:1438499 blocked for more than 120 seconds.
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  kworker/0:1 D 0 1438499 2 0x80000080
  Workqueue: cgroup_destroy css_free_rwork_fn
  ? __schedule+0x296/0x900
  schedule+0x28/0x80
  schedule_timeout+0x1ee/0x3a0
  ? kvm_sched_clock_read+0xd/0x20
  wait_for_completion+0x12c/0x190
  ? wake_up_q+0x70/0x70
  flush_workqueue+0x132/0x430
  ? cgroup1_pidlist_destroy_all+0x7c/0xa0
  cgroup1_pidlist_destroy_all+0x7c/0xa0
  css_free_rwork_fn+0xb5/0x390
  process_one_work+0x195/0x3e0
  worker_thread+0x30/0x390
  ? process_one_work+0x3e0/0x3e0
  kthread+0x113/0x130
  ? kthread_create_worker_on_cpu+0x70/0x70
  ret_from_fork+0x1f/0x40

Fix it by moving the cgroup's pidlists to a local orphan list under
pidlist_mutex, clearing their ->owner pointer, and then cancelling each
pidlist's delayed work outside the lock. The destroy work function now
checks ->owner and skips freeing orphaned pidlists, so
cgroup1_pidlist_destroy_all() can free them safely without flushing the
whole shared workqueue.

Signed-off-by: Junnan Zhang <zhangjn11@chinatelecom.cn>
Signed-off-by: Shouxin Sun <sunshx@chinatelecom.cn>

---
v1 -> v2:
 - Fix a NULL pointer dereference in cgroup_pidlist_destroy_work_fn():
   use the cached owner pointer for mutex_unlock(), as ->owner may have
   been cleared concurrently by cgroup1_pidlist_destroy_all(). Spotted
   by Sashiko AI review.

v2 -> v3:
 - Use READ_ONCE()/WRITE_ONCE() for the ->owner field shared between
   cgroup1_pidlist_destroy_all() and cgroup_pidlist_destroy_work_fn() to
   prevent data races. Spotted by Sashiko AI review.
---
 kernel/cgroup/cgroup-v1.c | 53 ++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 15 deletions(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index a4337c9b5287..9abe8b791bb1 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -206,13 +206,31 @@ struct cgroup_pidlist {
 void cgroup1_pidlist_destroy_all(struct cgroup *cgrp)
 {
 	struct cgroup_pidlist *l, *tmp_l;
+	LIST_HEAD(orphan);
+
+	/*
+	 * Move pidlists to a local orphan list and mark them as owner-less.
+	 * The destroy work function will see ->owner == NULL and skip freeing.
+	 * We then cancel and free them outside pidlist_mutex to avoid
+	 * flush_workqueue() blocking on the shared workqueue.
+	 */
 
 	mutex_lock(&cgrp->pidlist_mutex);
-	list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links)
-		mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork, 0);
+	list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links) {
+		list_del(&l->links);
+		WRITE_ONCE(l->owner, NULL);
+		list_add(&l->links, &orphan);
+	}
 	mutex_unlock(&cgrp->pidlist_mutex);
 
-	flush_workqueue(cgroup_pidlist_destroy_wq);
+	list_for_each_entry_safe(l, tmp_l, &orphan, links) {
+		list_del(&l->links);
+		cancel_delayed_work_sync(&l->destroy_dwork);
+		kvfree(l->list);
+		put_pid_ns(l->key.ns);
+		kfree(l);
+	}
+
 	BUG_ON(!list_empty(&cgrp->pidlists));
 }
 
@@ -222,21 +240,26 @@ static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
 	struct cgroup_pidlist *l = container_of(dwork, struct cgroup_pidlist,
 						destroy_dwork);
 	struct cgroup_pidlist *tofree = NULL;
+	struct cgroup *owner;
 
-	mutex_lock(&l->owner->pidlist_mutex);
+	owner = READ_ONCE(l->owner);
+	if (owner) {
+		mutex_lock(&owner->pidlist_mutex);
 
-	/*
-	 * Destroy iff we didn't get queued again.  The state won't change
-	 * as destroy_dwork can only be queued while locked.
-	 */
-	if (!delayed_work_pending(dwork)) {
-		list_del(&l->links);
-		kvfree(l->list);
-		put_pid_ns(l->key.ns);
-		tofree = l;
-	}
+		/*
+		 * Destroy iff we didn't get queued again and we're still
+		 * owned by the cgroup.  If ->owner was cleared by
+		 * cgroup1_pidlist_destroy_all(), it will free us.
+		 */
+		if (READ_ONCE(l->owner) == owner && !delayed_work_pending(dwork)) {
+			list_del(&l->links);
+			kvfree(l->list);
+			put_pid_ns(l->key.ns);
+			tofree = l;
+		}
 
-	mutex_unlock(&l->owner->pidlist_mutex);
+		mutex_unlock(&owner->pidlist_mutex);
+	}
 	kfree(tofree);
 }
 
-- 
2.43.0

Re: [PATCH v3] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Junnan Zhang]

Gentle ping.

It's been a couple of weeks since v3, and there hasn't been any
maintainer feedback yet. Could you take a look when you have a
chance? Happy to address any comments or rebase if needed.

Thanks,
Junnan

Re: [PATCH v3] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Michal Koutný]

[-- Attachment #1: Type: text/plain, Size: 3099 bytes --]

Hi Junnan.

(Sorry for late response, I sketched some notes and then didn't get down
to sent them. Now they're below.)

On Fri, Aug 14, 2026 at 06:20:52PM +0800, Junnan Zhang <zhangjn_dev@163.com> wrote:
> From: Junnan Zhang <zhangjn11@chinatelecom.cn>
> 
> cgroup1_pidlist_destroy_all() flushes the global
> cgroup_pidlist_destroy_wq while destroying a cgroup. Because all cgroup
> v1 pidlist destruction work items are queued on the same shared workqueue,
> a single slow or stuck work item (e.g. waiting for pidlist_mutex held by a
> user-space reader) blocks every concurrent cgroup destruction path.
> 
> This can lead to kworker tasks stuck in flush_workqueue() for over
> hung_task_timeout seconds, as observed on busy systems running Docker or
> Kubernetes workloads.

Since the cgroup_pidlist_destroy_wq is already a dedicated workqueue (no
other conteders), the pursuit of pidlist_mutex holder is a feasible
theory. However, that would also mean:
a) a single reader taking more than hung_task_timeout_secs (that'd be
   a softlockup earlier),
b) starvation of cgroup1_pidlist_destroy_all() by many (queued)
   cgroup_pidlist_start() callers which goes against the second-long
   caching of pidlists,
c) there is large number of nr_cgroups * nr_pidnses which makes the
   caching ineffective

> 
>   INFO: task kworker/0:1:1438499 blocked for more than 120 seconds.
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   kworker/0:1 D 0 1438499 2 0x80000080
>   Workqueue: cgroup_destroy css_free_rwork_fn
>   ? __schedule+0x296/0x900
>   schedule+0x28/0x80
>   schedule_timeout+0x1ee/0x3a0
>   ? kvm_sched_clock_read+0xd/0x20
>   wait_for_completion+0x12c/0x190
>   ? wake_up_q+0x70/0x70
>   flush_workqueue+0x132/0x430
>   ? cgroup1_pidlist_destroy_all+0x7c/0xa0
>   cgroup1_pidlist_destroy_all+0x7c/0xa0
>   css_free_rwork_fn+0xb5/0x390
>   process_one_work+0x195/0x3e0
>   worker_thread+0x30/0x390
>   ? process_one_work+0x3e0/0x3e0
>   kthread+0x113/0x130
>   ? kthread_create_worker_on_cpu+0x70/0x70
>   ret_from_fork+0x1f/0x40
> 
> Fix it by moving the cgroup's pidlists to a local orphan list under
> pidlist_mutex, clearing their ->owner pointer, and then cancelling each
> pidlist's delayed work outside the lock. The destroy work function now
> checks ->owner and skips freeing orphaned pidlists, so
> cgroup1_pidlist_destroy_all() can free them safely without flushing the
> whole shared workqueue.

What's the point of the workqueue after this change? (Mainly the
expiration + having process context for the handler.)

The flushing isn't necessary if there's a way how to ensure pidlists
head won't be used after cgrp removal, which the fix should achieve.


So I'd say, the narrow-focused cancellation may work, no need to wait
for other cgroups. OTOH, I'm surprised this v1-issue popped up only now
and whether such a long contention can happen over pidlist_mutex as your
commit message implies. What nr_cgroups, nr_pidnses could cause this in
your theory?

Thanks,
Michal


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Re: [PATCH v3] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Ridong Chen]

On 8/31/2026 4:16 PM, Michal Koutný wrote:
> Hi Junnan.
> 
> (Sorry for late response, I sketched some notes and then didn't get down
> to sent them. Now they're below.)
> 
> On Fri, Aug 14, 2026 at 06:20:52PM +0800, Junnan Zhang <zhangjn_dev@163.com> wrote:
>> From: Junnan Zhang <zhangjn11@chinatelecom.cn>
>>
>> cgroup1_pidlist_destroy_all() flushes the global
>> cgroup_pidlist_destroy_wq while destroying a cgroup. Because all cgroup
>> v1 pidlist destruction work items are queued on the same shared workqueue,
>> a single slow or stuck work item (e.g. waiting for pidlist_mutex held by a
>> user-space reader) blocks every concurrent cgroup destruction path.
>>
>> This can lead to kworker tasks stuck in flush_workqueue() for over
>> hung_task_timeout seconds, as observed on busy systems running Docker or
>> Kubernetes workloads.
> 
> Since the cgroup_pidlist_destroy_wq is already a dedicated workqueue (no
> other conteders), the pursuit of pidlist_mutex holder is a feasible
> theory. However, that would also mean:
> a) a single reader taking more than hung_task_timeout_secs (that'd be
>     a softlockup earlier),
> b) starvation of cgroup1_pidlist_destroy_all() by many (queued)
>     cgroup_pidlist_start() callers which goes against the second-long
>     caching of pidlists,
> c) there is large number of nr_cgroups * nr_pidnses which makes the
>     caching ineffective
> 
>>
>>    INFO: task kworker/0:1:1438499 blocked for more than 120 seconds.
>>    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>    kworker/0:1 D 0 1438499 2 0x80000080
>>    Workqueue: cgroup_destroy css_free_rwork_fn
>>    ? __schedule+0x296/0x900
>>    schedule+0x28/0x80
>>    schedule_timeout+0x1ee/0x3a0
>>    ? kvm_sched_clock_read+0xd/0x20
>>    wait_for_completion+0x12c/0x190
>>    ? wake_up_q+0x70/0x70
>>    flush_workqueue+0x132/0x430
>>    ? cgroup1_pidlist_destroy_all+0x7c/0xa0
>>    cgroup1_pidlist_destroy_all+0x7c/0xa0
>>    css_free_rwork_fn+0xb5/0x390
>>    process_one_work+0x195/0x3e0
>>    worker_thread+0x30/0x390
>>    ? process_one_work+0x3e0/0x3e0
>>    kthread+0x113/0x130
>>    ? kthread_create_worker_on_cpu+0x70/0x70
>>    ret_from_fork+0x1f/0x40
>>

I'm not sure we've found the real root cause yet, and the current analysis 
doesn't convince me.

Shouldn't we first figure out why flush_workqueue() waited 120s? Was it because 
there were too many pids, or because someone held pidlist_mutex for too long?

>> Fix it by moving the cgroup's pidlists to a local orphan list under
>> pidlist_mutex, clearing their ->owner pointer, and then cancelling each
>> pidlist's delayed work outside the lock. The destroy work function now
>> checks ->owner and skips freeing orphaned pidlists, so
>> cgroup1_pidlist_destroy_all() can free them safely without flushing the
>> whole shared workqueue.
> 
> What's the point of the workqueue after this change? (Mainly the
> expiration + having process context for the handler.)
> 
> The flushing isn't necessary if there's a way how to ensure pidlists
> head won't be used after cgrp removal, which the fix should achieve.
> 
> 
> So I'd say, the narrow-focused cancellation may work, no need to wait
> for other cgroups. OTOH, I'm surprised this v1-issue popped up only now
> and whether such a long contention can happen over pidlist_mutex as your
> commit message implies. What nr_cgroups, nr_pidnses could cause this in
> your theory?
> 
> Thanks,
> Michal

-- 
Best regards
Ridong