The update_isolation_cpumasks() function can be called either directly
from regular cpuset control file write with cpuset_full_lock() called
or via the CPU hotplug path with cpus_write_lock and cpuset_mutex held.

As we are going to enable dynamic update to the nozh_full housekeeping
cpumask (HK_TYPE_KERNEL_NOISE) soon with the help of CPU hotplug,
allowing the CPU hotplug path to call into housekeeping_update()
directly from update_isolation_cpumasks() will cause deadlock. So we
have to defer any call to housekeeping_update() after the CPU hotplug
operation has finished. This can be done via the task_work_add(...,
TWA_RESUME) API where the actual housekeeping_update() call, if needed,
will happen right before existing back to userspace.

Since the HK_TYPE_DOMAIN housekeeping cpumask should now track the
changes in "cpuset.cpus.isolated", add a check in test_cpuset_prs.sh to
confirm that the CPU hotplug deferral, if needed, is working as expected.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c                        | 49 ++++++++++++++++++-
 .../selftests/cgroup/test_cpuset_prs.sh       |  9 ++++
 2 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7b7d12ab1006..98c7cb732206 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -84,6 +84,10 @@ static cpumask_var_t	isolated_cpus;
  */
 static bool isolated_cpus_updating;
 
+/* Both cpuset_mutex and cpus_read_locked acquired */
+static bool cpuset_full_locked;
+static bool isolation_task_work_queued;
+
 /*
  * A flag to force sched domain rebuild at the end of an operation.
  * It can be set in
@@ -285,10 +289,12 @@ void cpuset_full_lock(void)
 {
 	cpus_read_lock();
 	mutex_lock(&cpuset_mutex);
+	cpuset_full_locked = true;
 }
 
 void cpuset_full_unlock(void)
 {
+	cpuset_full_locked = false;
 	mutex_unlock(&cpuset_mutex);
 	cpus_read_unlock();
 }
@@ -1285,25 +1291,64 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
 	return false;
 }
 
+static void __update_isolation_cpumasks(bool twork);
+static void isolation_task_work_fn(struct callback_head *cb)
+{
+	cpuset_full_lock();
+	__update_isolation_cpumasks(true);
+	cpuset_full_lock();
+}
+
 /*
- * update_isolation_cpumasks - Update external isolation related CPU masks
+ * __update_isolation_cpumasks - Update external isolation related CPU masks
+ * @twork - set if call from isolation_task_work_fn()
  *
  * The following external CPU masks will be updated if necessary:
  * - workqueue unbound cpumask
  */
-static void update_isolation_cpumasks(void)
+static void __update_isolation_cpumasks(bool twork)
 {
 	int ret;
 
+	if (twork)
+		isolation_task_work_queued = false;
+
 	if (!isolated_cpus_updating)
 		return;
 
+	/*
+	 * This function can be reached either directly from regular cpuset
+	 * control file write (cpuset_full_locked) or via hotplug
+	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
+	 * defer the housekeeping_update() call to a task_work to avoid
+	 * the possibility of deadlock. The task_work will be run right
+	 * before exiting back to userspace.
+	 */
+	if (!cpuset_full_locked) {
+		static struct callback_head twork_cb;
+
+		if (!isolation_task_work_queued) {
+			init_task_work(&twork_cb, isolation_task_work_fn);
+			if (!task_work_add(current, &twork_cb, TWA_RESUME))
+				isolation_task_work_queued = true;
+			else
+				/* Current task shouldn't be exiting */
+				WARN_ON_ONCE(1);
+		}
+		return;
+	}
+
 	ret = housekeeping_update(isolated_cpus);
 	WARN_ON_ONCE(ret < 0);
 
 	isolated_cpus_updating = false;
 }
 
+static inline void update_isolation_cpumasks(void)
+{
+	__update_isolation_cpumasks(false);
+}
+
 /**
  * rm_siblings_excl_cpus - Remove exclusive CPUs that are used by sibling cpusets
  * @parent: Parent cpuset containing all siblings
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 5dff3ad53867..af4a2532cb3e 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -773,6 +773,7 @@ check_isolcpus()
 	EXPECTED_ISOLCPUS=$1
 	ISCPUS=${CGROUP2}/cpuset.cpus.isolated
 	ISOLCPUS=$(cat $ISCPUS)
+	HKICPUS=$(cat /sys/devices/system/cpu/isolated)
 	LASTISOLCPU=
 	SCHED_DOMAINS=/sys/kernel/debug/sched/domains
 	if [[ $EXPECTED_ISOLCPUS = . ]]
@@ -810,6 +811,14 @@ check_isolcpus()
 	ISOLCPUS=
 	EXPECTED_ISOLCPUS=$EXPECTED_SDOMAIN
 
+	#
+	# The inverse of HK_TYPE_DOMAIN cpumask in $HKICPUS should match $ISOLCPUS
+	#
+	[[ "$ISOLCPUS" != "$HKICPUS" ]] && {
+		echo "Housekeeping isolated CPUs mismatch - $HKICPUS"
+		return 1
+	}
+
 	#
 	# Use the sched domain in debugfs to check isolated CPUs, if available
 	#
-- 
2.52.0

On 2026/1/28 12:42, Waiman Long wrote:
> The update_isolation_cpumasks() function can be called either directly
> from regular cpuset control file write with cpuset_full_lock() called
> or via the CPU hotplug path with cpus_write_lock and cpuset_mutex held.
> 
> As we are going to enable dynamic update to the nozh_full housekeeping
> cpumask (HK_TYPE_KERNEL_NOISE) soon with the help of CPU hotplug,
> allowing the CPU hotplug path to call into housekeeping_update()
> directly from update_isolation_cpumasks() will cause deadlock. So we
> have to defer any call to housekeeping_update() after the CPU hotplug
> operation has finished. This can be done via the task_work_add(...,
> TWA_RESUME) API where the actual housekeeping_update() call, if needed,
> will happen right before existing back to userspace.
> 
> Since the HK_TYPE_DOMAIN housekeeping cpumask should now track the
> changes in "cpuset.cpus.isolated", add a check in test_cpuset_prs.sh to
> confirm that the CPU hotplug deferral, if needed, is working as expected.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c                        | 49 ++++++++++++++++++-
>  .../selftests/cgroup/test_cpuset_prs.sh       |  9 ++++
>  2 files changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 7b7d12ab1006..98c7cb732206 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -84,6 +84,10 @@ static cpumask_var_t	isolated_cpus;
>   */
>  static bool isolated_cpus_updating;
>  
> +/* Both cpuset_mutex and cpus_read_locked acquired */
> +static bool cpuset_full_locked;
> +static bool isolation_task_work_queued;
> +
>  /*
>   * A flag to force sched domain rebuild at the end of an operation.
>   * It can be set in
> @@ -285,10 +289,12 @@ void cpuset_full_lock(void)
>  {
>  	cpus_read_lock();
>  	mutex_lock(&cpuset_mutex);
> +	cpuset_full_locked = true;
>  }
>  
>  void cpuset_full_unlock(void)
>  {
> +	cpuset_full_locked = false;
>  	mutex_unlock(&cpuset_mutex);
>  	cpus_read_unlock();
>  }
> @@ -1285,25 +1291,64 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>  	return false;
>  }
>  
> +static void __update_isolation_cpumasks(bool twork);
> +static void isolation_task_work_fn(struct callback_head *cb)
> +{
> +	cpuset_full_lock();
> +	__update_isolation_cpumasks(true);
> +	cpuset_full_lock();
> +}
> +
>  /*
> - * update_isolation_cpumasks - Update external isolation related CPU masks
> + * __update_isolation_cpumasks - Update external isolation related CPU masks
> + * @twork - set if call from isolation_task_work_fn()
>   *
>   * The following external CPU masks will be updated if necessary:
>   * - workqueue unbound cpumask
>   */
> -static void update_isolation_cpumasks(void)
> +static void __update_isolation_cpumasks(bool twork)
>  {
>  	int ret;
>  
> +	if (twork)
> +		isolation_task_work_queued = false;
> +
>  	if (!isolated_cpus_updating)
>  		return;
>  
> +	/*
> +	 * This function can be reached either directly from regular cpuset
> +	 * control file write (cpuset_full_locked) or via hotplug
> +	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
> +	 * defer the housekeeping_update() call to a task_work to avoid
> +	 * the possibility of deadlock. The task_work will be run right
> +	 * before exiting back to userspace.
> +	 */
> +	if (!cpuset_full_locked) {
> +		static struct callback_head twork_cb;
> +
> +		if (!isolation_task_work_queued) {
> +			init_task_work(&twork_cb, isolation_task_work_fn);
> +			if (!task_work_add(current, &twork_cb, TWA_RESUME))
> +				isolation_task_work_queued = true;
> +			else
> +				/* Current task shouldn't be exiting */
> +				WARN_ON_ONCE(1);
> +		}
> +		return;
> +	}
> +
>  	ret = housekeeping_update(isolated_cpus);
>  	WARN_ON_ONCE(ret < 0);
>  
>  	isolated_cpus_updating = false;
>  }
>  

The logic is not straightforward; perhaps we can simplify it as follows,
maybe I missed something, just correct me.

static void isolation_task_work_fn(struct callback_head *cb)
{
	guard(mutex)(&isolcpus_update_mutex);
	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
}

/*
 * __update_isolation_cpumasks - Update external isolation related CPU masks
 * @twork - set if call from isolation_task_work_fn()
 *
 * The following external CPU masks will be updated if necessary:
 * - workqueue unbound cpumask
 */
static void __update_isolation_cpumasks(bool twork)
{
	if (!isolated_cpus_updating)
		return;

	/*
	 * This function can be reached either directly from regular cpuset
	 * control file write (cpuset_full_locked) or via hotplug
	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
	 * defer the housekeeping_update() call to a task_work to avoid
	 * the possibility of deadlock. The task_work will be run right
	 * before exiting back to userspace.
	 */
	if (twork) {
		static struct callback_head twork_cb;

		init_task_work(&twork_cb, isolation_task_work_fn);
		if (task_work_add(current, &twork_cb, TWA_RESUME))
			/* Current task shouldn't be exiting */
			WARN_ON_ONCE(1);

		return;
	}

	lockdep_assert_held(&isolcpus_update_mutex);
	/*
	 * Release cpus_read_lock & cpuset_mutex before calling
	 * housekeeping_update() and re-acquiring them afterward if not
	 * calling from task_work.
	 */

	cpuset_full_unlock();
	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
	cpuset_full_lock();

	isolated_cpus_updating = false;
}

static inline void update_isolation_cpumasks(void)
{
	__update_isolation_cpumasks(false);
}

If twork is true, add it directly to the work queue — note that this requires
holding isolcpus_update_mutex and then calling housekeeping_update. Otherwise,
release the 'full' lock before invoking housekeeping_update.

> +static inline void update_isolation_cpumasks(void)
> +{
> +	__update_isolation_cpumasks(false);
> +}
> +
>  /**
>   * rm_siblings_excl_cpus - Remove exclusive CPUs that are used by sibling cpusets
>   * @parent: Parent cpuset containing all siblings
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 5dff3ad53867..af4a2532cb3e 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -773,6 +773,7 @@ check_isolcpus()
>  	EXPECTED_ISOLCPUS=$1
>  	ISCPUS=${CGROUP2}/cpuset.cpus.isolated
>  	ISOLCPUS=$(cat $ISCPUS)
> +	HKICPUS=$(cat /sys/devices/system/cpu/isolated)
>  	LASTISOLCPU=
>  	SCHED_DOMAINS=/sys/kernel/debug/sched/domains
>  	if [[ $EXPECTED_ISOLCPUS = . ]]
> @@ -810,6 +811,14 @@ check_isolcpus()
>  	ISOLCPUS=
>  	EXPECTED_ISOLCPUS=$EXPECTED_SDOMAIN
>  
> +	#
> +	# The inverse of HK_TYPE_DOMAIN cpumask in $HKICPUS should match $ISOLCPUS
> +	#
> +	[[ "$ISOLCPUS" != "$HKICPUS" ]] && {
> +		echo "Housekeeping isolated CPUs mismatch - $HKICPUS"
> +		return 1
> +	}
> +
>  	#
>  	# Use the sched domain in debugfs to check isolated CPUs, if available
>  	#

-- 
Best regards,
Ridong

On 2026/1/29 12:03, Chen Ridong wrote:
> 
> 
> On 2026/1/28 12:42, Waiman Long wrote:
>> The update_isolation_cpumasks() function can be called either directly
>> from regular cpuset control file write with cpuset_full_lock() called
>> or via the CPU hotplug path with cpus_write_lock and cpuset_mutex held.
>>
>> As we are going to enable dynamic update to the nozh_full housekeeping
>> cpumask (HK_TYPE_KERNEL_NOISE) soon with the help of CPU hotplug,
>> allowing the CPU hotplug path to call into housekeeping_update()
>> directly from update_isolation_cpumasks() will cause deadlock. So we
>> have to defer any call to housekeeping_update() after the CPU hotplug
>> operation has finished. This can be done via the task_work_add(...,
>> TWA_RESUME) API where the actual housekeeping_update() call, if needed,
>> will happen right before existing back to userspace.
>>
>> Since the HK_TYPE_DOMAIN housekeeping cpumask should now track the
>> changes in "cpuset.cpus.isolated", add a check in test_cpuset_prs.sh to
>> confirm that the CPU hotplug deferral, if needed, is working as expected.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  kernel/cgroup/cpuset.c                        | 49 ++++++++++++++++++-
>>  .../selftests/cgroup/test_cpuset_prs.sh       |  9 ++++
>>  2 files changed, 56 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 7b7d12ab1006..98c7cb732206 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -84,6 +84,10 @@ static cpumask_var_t	isolated_cpus;
>>   */
>>  static bool isolated_cpus_updating;
>>  
>> +/* Both cpuset_mutex and cpus_read_locked acquired */
>> +static bool cpuset_full_locked;
>> +static bool isolation_task_work_queued;
>> +
>>  /*
>>   * A flag to force sched domain rebuild at the end of an operation.
>>   * It can be set in
>> @@ -285,10 +289,12 @@ void cpuset_full_lock(void)
>>  {
>>  	cpus_read_lock();
>>  	mutex_lock(&cpuset_mutex);
>> +	cpuset_full_locked = true;
>>  }
>>  
>>  void cpuset_full_unlock(void)
>>  {
>> +	cpuset_full_locked = false;
>>  	mutex_unlock(&cpuset_mutex);
>>  	cpus_read_unlock();
>>  }
>> @@ -1285,25 +1291,64 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>>  	return false;
>>  }
>>  
>> +static void __update_isolation_cpumasks(bool twork);
>> +static void isolation_task_work_fn(struct callback_head *cb)
>> +{
>> +	cpuset_full_lock();
>> +	__update_isolation_cpumasks(true);
>> +	cpuset_full_lock();
>> +}
>> +
>>  /*
>> - * update_isolation_cpumasks - Update external isolation related CPU masks
>> + * __update_isolation_cpumasks - Update external isolation related CPU masks
>> + * @twork - set if call from isolation_task_work_fn()
>>   *
>>   * The following external CPU masks will be updated if necessary:
>>   * - workqueue unbound cpumask
>>   */
>> -static void update_isolation_cpumasks(void)
>> +static void __update_isolation_cpumasks(bool twork)
>>  {
>>  	int ret;
>>  
>> +	if (twork)
>> +		isolation_task_work_queued = false;
>> +
>>  	if (!isolated_cpus_updating)
>>  		return;
>>  
>> +	/*
>> +	 * This function can be reached either directly from regular cpuset
>> +	 * control file write (cpuset_full_locked) or via hotplug
>> +	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
>> +	 * defer the housekeeping_update() call to a task_work to avoid
>> +	 * the possibility of deadlock. The task_work will be run right
>> +	 * before exiting back to userspace.
>> +	 */
>> +	if (!cpuset_full_locked) {
>> +		static struct callback_head twork_cb;
>> +
>> +		if (!isolation_task_work_queued) {
>> +			init_task_work(&twork_cb, isolation_task_work_fn);
>> +			if (!task_work_add(current, &twork_cb, TWA_RESUME))
>> +				isolation_task_work_queued = true;
>> +			else
>> +				/* Current task shouldn't be exiting */
>> +				WARN_ON_ONCE(1);
>> +		}
>> +		return;
>> +	}
>> +
>>  	ret = housekeeping_update(isolated_cpus);
>>  	WARN_ON_ONCE(ret < 0);
>>  
>>  	isolated_cpus_updating = false;
>>  }
>>  
> 
> The logic is not straightforward; perhaps we can simplify it as follows,
> maybe I missed something, just correct me.
> 
> static void isolation_task_work_fn(struct callback_head *cb)
> {
> 	guard(mutex)(&isolcpus_update_mutex);
> 	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
> }
> 
> /*
>  * __update_isolation_cpumasks - Update external isolation related CPU masks
>  * @twork - set if call from isolation_task_work_fn()
>  *
>  * The following external CPU masks will be updated if necessary:
>  * - workqueue unbound cpumask
>  */
> static void __update_isolation_cpumasks(bool twork)
> {
> 	if (!isolated_cpus_updating)
> 		return;
> 
> 	/*
> 	 * This function can be reached either directly from regular cpuset
> 	 * control file write (cpuset_full_locked) or via hotplug
> 	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
> 	 * defer the housekeeping_update() call to a task_work to avoid
> 	 * the possibility of deadlock. The task_work will be run right
> 	 * before exiting back to userspace.
> 	 */
> 	if (twork) {
> 		static struct callback_head twork_cb;
> 
> 		init_task_work(&twork_cb, isolation_task_work_fn);
> 		if (task_work_add(current, &twork_cb, TWA_RESUME))
> 			/* Current task shouldn't be exiting */
> 			WARN_ON_ONCE(1);
> 
> 		return;
> 	}
> 
> 	lockdep_assert_held(&isolcpus_update_mutex);
> 	/*
> 	 * Release cpus_read_lock & cpuset_mutex before calling
> 	 * housekeeping_update() and re-acquiring them afterward if not
> 	 * calling from task_work.
> 	 */
> 
> 	cpuset_full_unlock();
> 	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
> 	cpuset_full_lock();
> 
> 	isolated_cpus_updating = false;
> }
> 
> static inline void update_isolation_cpumasks(void)
> {
> 	__update_isolation_cpumasks(false);
> }
> 

It can be much clearer:

static void isolation_task_work_fn(struct callback_head *cb)
{
	guard(mutex)(&isolcpus_update_mutex);
	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
}

/*
 * __update_isolation_cpumasks - Update external isolation related CPU masks
 * @defer
 *
 * The following external CPU masks will be updated if necessary:
 * - workqueue unbound cpumask
 */
static void __update_isolation_cpumasks(bool defer)
{
	if (!isolated_cpus_updating)
		return;

	/*
	 * This function can be reached either directly from regular cpuset
	 * control file write (cpuset_full_locked) or via hotplug
	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
	 * defer the housekeeping_update() call to a task_work to avoid
	 * the possibility of deadlock. The task_work will be run right
	 * before exiting back to userspace.
	 */
	if (defer) {
		static struct callback_head twork_cb;

		init_task_work(&twork_cb, isolation_task_work_fn);
		if (task_work_add(current, &twork_cb, TWA_RESUME))
			/* Current task shouldn't be exiting */
			WARN_ON_ONCE(1);

		return;
	}

	lockdep_assert_held(&isolcpus_update_mutex);
	lockdep_assert_cpus_held();
	lockdep_assert_cpuset_lock_held();

	/*
	 * Release cpus_read_lock & cpuset_mutex before calling
	 * housekeeping_update() and re-acquiring them afterward if not
	 * calling from task_work.
	 */

	cpuset_full_unlock();
	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
	cpuset_full_lock();

	isolated_cpus_updating = false;
}

static inline void update_isolation_cpumasks(void)
{
	__update_isolation_cpumasks(false);
}

static inline void asyn_update_isolation_cpumasks(void)
{
	__update_isolation_cpumasks(true);
}

The hotplug path just calls asyn_update_isolation_cpumasks(), cpuset_full_locked
and isolation_task_work_queued can be removed.

-- 
Best regards,
Ridong

On 1/29/26 2:15 AM, Chen Ridong wrote:
>
> On 2026/1/29 12:03, Chen Ridong wrote:
>>
>> On 2026/1/28 12:42, Waiman Long wrote:
>>> The update_isolation_cpumasks() function can be called either directly
>>> from regular cpuset control file write with cpuset_full_lock() called
>>> or via the CPU hotplug path with cpus_write_lock and cpuset_mutex held.
>>>
>>> As we are going to enable dynamic update to the nozh_full housekeeping
>>> cpumask (HK_TYPE_KERNEL_NOISE) soon with the help of CPU hotplug,
>>> allowing the CPU hotplug path to call into housekeeping_update()
>>> directly from update_isolation_cpumasks() will cause deadlock. So we
>>> have to defer any call to housekeeping_update() after the CPU hotplug
>>> operation has finished. This can be done via the task_work_add(...,
>>> TWA_RESUME) API where the actual housekeeping_update() call, if needed,
>>> will happen right before existing back to userspace.
>>>
>>> Since the HK_TYPE_DOMAIN housekeeping cpumask should now track the
>>> changes in "cpuset.cpus.isolated", add a check in test_cpuset_prs.sh to
>>> confirm that the CPU hotplug deferral, if needed, is working as expected.
>>>
>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>> ---
>>>   kernel/cgroup/cpuset.c                        | 49 ++++++++++++++++++-
>>>   .../selftests/cgroup/test_cpuset_prs.sh       |  9 ++++
>>>   2 files changed, 56 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>> index 7b7d12ab1006..98c7cb732206 100644
>>> --- a/kernel/cgroup/cpuset.c
>>> +++ b/kernel/cgroup/cpuset.c
>>> @@ -84,6 +84,10 @@ static cpumask_var_t	isolated_cpus;
>>>    */
>>>   static bool isolated_cpus_updating;
>>>   
>>> +/* Both cpuset_mutex and cpus_read_locked acquired */
>>> +static bool cpuset_full_locked;
>>> +static bool isolation_task_work_queued;
>>> +
>>>   /*
>>>    * A flag to force sched domain rebuild at the end of an operation.
>>>    * It can be set in
>>> @@ -285,10 +289,12 @@ void cpuset_full_lock(void)
>>>   {
>>>   	cpus_read_lock();
>>>   	mutex_lock(&cpuset_mutex);
>>> +	cpuset_full_locked = true;
>>>   }
>>>   
>>>   void cpuset_full_unlock(void)
>>>   {
>>> +	cpuset_full_locked = false;
>>>   	mutex_unlock(&cpuset_mutex);
>>>   	cpus_read_unlock();
>>>   }
>>> @@ -1285,25 +1291,64 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>>>   	return false;
>>>   }
>>>   
>>> +static void __update_isolation_cpumasks(bool twork);
>>> +static void isolation_task_work_fn(struct callback_head *cb)
>>> +{
>>> +	cpuset_full_lock();
>>> +	__update_isolation_cpumasks(true);
>>> +	cpuset_full_lock();
>>> +}
>>> +
>>>   /*
>>> - * update_isolation_cpumasks - Update external isolation related CPU masks
>>> + * __update_isolation_cpumasks - Update external isolation related CPU masks
>>> + * @twork - set if call from isolation_task_work_fn()
>>>    *
>>>    * The following external CPU masks will be updated if necessary:
>>>    * - workqueue unbound cpumask
>>>    */
>>> -static void update_isolation_cpumasks(void)
>>> +static void __update_isolation_cpumasks(bool twork)
>>>   {
>>>   	int ret;
>>>   
>>> +	if (twork)
>>> +		isolation_task_work_queued = false;
>>> +
>>>   	if (!isolated_cpus_updating)
>>>   		return;
>>>   
>>> +	/*
>>> +	 * This function can be reached either directly from regular cpuset
>>> +	 * control file write (cpuset_full_locked) or via hotplug
>>> +	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
>>> +	 * defer the housekeeping_update() call to a task_work to avoid
>>> +	 * the possibility of deadlock. The task_work will be run right
>>> +	 * before exiting back to userspace.
>>> +	 */
>>> +	if (!cpuset_full_locked) {
>>> +		static struct callback_head twork_cb;
>>> +
>>> +		if (!isolation_task_work_queued) {
>>> +			init_task_work(&twork_cb, isolation_task_work_fn);
>>> +			if (!task_work_add(current, &twork_cb, TWA_RESUME))
>>> +				isolation_task_work_queued = true;
>>> +			else
>>> +				/* Current task shouldn't be exiting */
>>> +				WARN_ON_ONCE(1);
>>> +		}
>>> +		return;
>>> +	}
>>> +
>>>   	ret = housekeeping_update(isolated_cpus);
>>>   	WARN_ON_ONCE(ret < 0);
>>>   
>>>   	isolated_cpus_updating = false;
>>>   }
>>>   
>> The logic is not straightforward; perhaps we can simplify it as follows,
>> maybe I missed something, just correct me.
>>
>> static void isolation_task_work_fn(struct callback_head *cb)
>> {
>> 	guard(mutex)(&isolcpus_update_mutex);
>> 	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>> }
>>
>> /*
>>   * __update_isolation_cpumasks - Update external isolation related CPU masks
>>   * @twork - set if call from isolation_task_work_fn()
>>   *
>>   * The following external CPU masks will be updated if necessary:
>>   * - workqueue unbound cpumask
>>   */
>> static void __update_isolation_cpumasks(bool twork)
>> {
>> 	if (!isolated_cpus_updating)
>> 		return;
>>
>> 	/*
>> 	 * This function can be reached either directly from regular cpuset
>> 	 * control file write (cpuset_full_locked) or via hotplug
>> 	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
>> 	 * defer the housekeeping_update() call to a task_work to avoid
>> 	 * the possibility of deadlock. The task_work will be run right
>> 	 * before exiting back to userspace.
>> 	 */
>> 	if (twork) {
>> 		static struct callback_head twork_cb;
>>
>> 		init_task_work(&twork_cb, isolation_task_work_fn);
>> 		if (task_work_add(current, &twork_cb, TWA_RESUME))
>> 			/* Current task shouldn't be exiting */
>> 			WARN_ON_ONCE(1);
>>
>> 		return;
>> 	}
>>
>> 	lockdep_assert_held(&isolcpus_update_mutex);
>> 	/*
>> 	 * Release cpus_read_lock & cpuset_mutex before calling
>> 	 * housekeeping_update() and re-acquiring them afterward if not
>> 	 * calling from task_work.
>> 	 */
>>
>> 	cpuset_full_unlock();
>> 	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>> 	cpuset_full_lock();
>>
>> 	isolated_cpus_updating = false;
>> }
>>
>> static inline void update_isolation_cpumasks(void)
>> {
>> 	__update_isolation_cpumasks(false);
>> }
>>
> It can be much clearer:
>
> static void isolation_task_work_fn(struct callback_head *cb)
> {
> 	guard(mutex)(&isolcpus_update_mutex);
> 	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
> }
>
> /*
>   * __update_isolation_cpumasks - Update external isolation related CPU masks
>   * @defer
>   *
>   * The following external CPU masks will be updated if necessary:
>   * - workqueue unbound cpumask
>   */
> static void __update_isolation_cpumasks(bool defer)
> {
> 	if (!isolated_cpus_updating)
> 		return;
>
> 	/*
> 	 * This function can be reached either directly from regular cpuset
> 	 * control file write (cpuset_full_locked) or via hotplug
> 	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
> 	 * defer the housekeeping_update() call to a task_work to avoid
> 	 * the possibility of deadlock. The task_work will be run right
> 	 * before exiting back to userspace.
> 	 */
> 	if (defer) {
> 		static struct callback_head twork_cb;
>
> 		init_task_work(&twork_cb, isolation_task_work_fn);
> 		if (task_work_add(current, &twork_cb, TWA_RESUME))
> 			/* Current task shouldn't be exiting */
> 			WARN_ON_ONCE(1);
>
> 		return;
> 	}
>
> 	lockdep_assert_held(&isolcpus_update_mutex);
> 	lockdep_assert_cpus_held();
> 	lockdep_assert_cpuset_lock_held();
>
> 	/*
> 	 * Release cpus_read_lock & cpuset_mutex before calling
> 	 * housekeeping_update() and re-acquiring them afterward if not
> 	 * calling from task_work.
> 	 */
>
> 	cpuset_full_unlock();
> 	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
> 	cpuset_full_lock();
>
> 	isolated_cpus_updating = false;
> }
>
> static inline void update_isolation_cpumasks(void)
> {
> 	__update_isolation_cpumasks(false);
> }
>
> static inline void asyn_update_isolation_cpumasks(void)
> {
> 	__update_isolation_cpumasks(true);
> }
>
> The hotplug path just calls asyn_update_isolation_cpumasks(), cpuset_full_locked
> and isolation_task_work_queued can be removed.
>
Thanks for the suggestions. I will adopt some of them. BTW, I am 
switching to use workqueue instead of the task_work as the latter is 
suitable in this use case.

Cheers,
Longman

On 1/29/26 8:37 PM, Waiman Long wrote:
>> The hotplug path just calls asyn_update_isolation_cpumasks(), 
>> cpuset_full_locked
>> and isolation_task_work_queued can be removed.
>>
> Thanks for the suggestions. I will adopt some of them. BTW, I am 
> switching to use workqueue instead of the task_work as the latter is 
> suitable in this use case.

Sorry, I mean "the latter is NOT suitable in this use case."

Cheers,
Longman

On Tue, Jan 27, 2026 at 11:42:50PM -0500, Waiman Long wrote:
> +static void isolation_task_work_fn(struct callback_head *cb)
> +{
> +	cpuset_full_lock();
> +	__update_isolation_cpumasks(true);
> +	cpuset_full_lock();
                    ^
                    unlock?

Thanks.

-- 
tejun

On 1/28/26 12:44 PM, Tejun Heo wrote:
> On Tue, Jan 27, 2026 at 11:42:50PM -0500, Waiman Long wrote:
>> +static void isolation_task_work_fn(struct callback_head *cb)
>> +{
>> +	cpuset_full_lock();
>> +	__update_isolation_cpumasks(true);
>> +	cpuset_full_lock();
>                      ^
>                      unlock?
>
> Thanks.
>
Sorry about that, will fix it. That change was removed in the 2nd patch. 
That is why the series still worked as expected.

Thanks,
Longman