Message-ID: <8fb3aab5-ba0a-4523-a404-5643b8a749c9@huaweicloud.com>
Date: Thu, 29 Jan 2026 16:20:51 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Shuah Khan <shuah@...nel.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH/for-next 2/2] cgroup/cpuset: Introduce a new top level
 isolcpus_update_mutex



On 2026/1/29 16:01, Chen Ridong wrote:
> 
> 
> On 2026/1/28 12:42, Waiman Long wrote:
>> The current cpuset partition code is able to dynamically update
>> the sched domains of a running system and the corresponding
>> HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the
>> "isolcpus=domain,..." boot command line feature at run time.
>>
>> The housekeeping cpumask update requires flushing a number of different
>> workqueues which may not be safe with cpus_read_lock() held as the
>> workqueue flushing code may acquire cpus_read_lock() or acquiring locks
>> which have locking dependency with cpus_read_lock() down the chain. Below
>> is an example of such circular locking problem.
>>
>>   ======================================================
>>   WARNING: possible circular locking dependency detected
>>   6.18.0-test+ #2 Tainted: G S
>>   ------------------------------------------------------
>>   test_cpuset_prs/10971 is trying to acquire lock:
>>   ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x7a/0x180
>>
>>   but task is already holding lock:
>>   ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130
>>
>>   which lock already depends on the new lock.
>>
>>   the existing dependency chain (in reverse order) is:
>>   -> #4 (cpuset_mutex){+.+.}-{4:4}:
>>   -> #3 (cpu_hotplug_lock){++++}-{0:0}:
>>   -> #2 (rtnl_mutex){+.+.}-{4:4}:
>>   -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}:
>>   -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}:
>>
>>   Chain exists of:
>>     (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex
>>
>>   5 locks held by test_cpuset_prs/10971:
>>    #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0xf9/0x1d0
>>    #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x260/0x5f0
>>    #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b6/0x5f0
>>    #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_partition_write+0x77/0x130
>>    #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130
>>
>>   Call Trace:
>>    <TASK>
>>      :
>>    touch_wq_lockdep_map+0x93/0x180
>>    __flush_workqueue+0x111/0x10b0
>>    housekeeping_update+0x12d/0x2d0
>>    update_parent_effective_cpumask+0x595/0x2440
>>    update_prstate+0x89d/0xce0
>>    cpuset_partition_write+0xc5/0x130
>>    cgroup_file_write+0x1a5/0x680
>>    kernfs_fop_write_iter+0x3df/0x5f0
>>    vfs_write+0x525/0xfd0
>>    ksys_write+0xf9/0x1d0
>>    do_syscall_64+0x95/0x520
>>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>
>> To avoid such a circular locking dependency problem, we have to
>> call housekeeping_update() without holding the cpus_read_lock()
>> and cpuset_mutex. One way to do that is to introduce a new top level
>> isolcpus_update_mutex which will be acquired first if the set of isolated
>> CPUs may have to be updated. This new isolcpus_update_mutex will provide
>> the need mutual exclusion without the need to hold cpus_read_lock().
>>

When I reviewed Frederic's patches, I concerned about this issue. However, I was
not certain whether any flush worker would need to acquire cpu_hotplug_lock or
cpuset_mutex.

Despite this warning, I do not understand how wq_completion would need to
acquire cpu_hotplug_lock and cpuset_mutex.

The reason I want to understand how wq_completion acquires cpu_hotplug_lock or
cpuset_mutex is to determine whether isolcpus_update_mutex is truly necessary.
As I mentioned in my previous email, I am concerned about a potential
use-after-free (UAF) issue, which might imply that isolcpus_update_mutex is
required in most places that currently acquire cpuset_mutex, with the possible
exception of the hotplug path?

>> As cpus_read_lock() is now no longer held when
>> tmigr_isolated_exclude_cpumask() is called, it needs to acquire it
>> directly.
>>
>> The lockdep_is_cpuset_held() is also updated to check the new
>> isolcpus_update_mutex.
>>
> 
> I worry about the issue:
> 
> CPU1				CPU2
> rmdir
> css->ss->css_killed(css);			
> cpuset_css_killed
> 				__update_isolation_cpumasks
> 				cpuset_full_unlock
> css->flags |= CSS_DYING;
> css_clear_dir(css);
> ...
> // offline and free do not
> // get isolcpus_update_mutex
> cpuset_css_offline
> cpuset_css_free
> 				cpuset_full_lock
> 				...
> 				// UAF?
> 
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>  kernel/cgroup/cpuset.c        | 79 ++++++++++++++++++++++++-----------
>>  kernel/sched/isolation.c      |  4 +-
>>  kernel/time/timer_migration.c |  3 +-
>>  3 files changed, 57 insertions(+), 29 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 98c7cb732206..96390ceb5122 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -78,7 +78,7 @@ static cpumask_var_t	subpartitions_cpus;
>>  static cpumask_var_t	isolated_cpus;
>>  
>>  /*
>> - * isolated_cpus updating flag (protected by cpuset_mutex)
>> + * isolated_cpus updating flag (protected by isolcpus_update_mutex)
>>   * Set if isolated_cpus is going to be updated in the current
>>   * cpuset_mutex crtical section.
>>   */
>> @@ -223,29 +223,46 @@ struct cpuset top_cpuset = {
>>  };
>>  
>>  /*
>> - * There are two global locks guarding cpuset structures - cpuset_mutex and
>> - * callback_lock. The cpuset code uses only cpuset_mutex. Other kernel
>> - * subsystems can use cpuset_lock()/cpuset_unlock() to prevent change to cpuset
>> - * structures. Note that cpuset_mutex needs to be a mutex as it is used in
>> - * paths that rely on priority inheritance (e.g. scheduler - on RT) for
>> - * correctness.
>> + * CPUSET Locking Convention
>> + * -------------------------
>>   *
>> - * A task must hold both locks to modify cpusets.  If a task holds
>> - * cpuset_mutex, it blocks others, ensuring that it is the only task able to
>> - * also acquire callback_lock and be able to modify cpusets.  It can perform
>> - * various checks on the cpuset structure first, knowing nothing will change.
>> - * It can also allocate memory while just holding cpuset_mutex.  While it is
>> - * performing these checks, various callback routines can briefly acquire
>> - * callback_lock to query cpusets.  Once it is ready to make the changes, it
>> - * takes callback_lock, blocking everyone else.
>> + * Below are the three global locks guarding cpuset structures in lock
>> + * acquisition order:
>> + *  - isolcpus_update_mutex (optional)
>> + *  - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
>> + *  - cpuset_mutex
>> + *  - callback_lock (raw spinlock)
>>   *
>> - * Calls to the kernel memory allocator can not be made while holding
>> - * callback_lock, as that would risk double tripping on callback_lock
>> - * from one of the callbacks into the cpuset code from within
>> - * __alloc_pages().
>> + * The first isolcpus_update_mutex should only be held if the existing set of
>> + * isolated CPUs (in isolated partition) or any of the partition states may be
>> + * changed when some cpuset control files are being written into. Otherwise,
>> + * it can be skipped. Holding isolcpus_update_mutex/cpus_read_lock or
>> + * cpus_write_lock will ensure mutual exclusion of isolated_cpus update.
>>   *
>> - * If a task is only holding callback_lock, then it has read-only
>> - * access to cpusets.
>> + * As cpuset will now indirectly flush a number of different workqueues in
>> + * housekeeping_update() when the set of isolated CPUs is going to be changed,
>> + * it may not be safe from the circular locking perspective to hold the
>> + * cpus_read_lock. So cpuset_full_lock() will be released before calling
>> + * housekeeping_update() and re-acquired afterward.
>> + *
>> + * A task must hold all the remaining three locks to modify externally visible
>> + * or used fields of cpusets, though some of the internally used cpuset fields
>> + * can be modified by holding cpu_hotplug_lock and cpuset_mutex only. If only
>> + * reliable read access of the externally used fields are needed, a task can
>> + * hold either cpuset_mutex or callback_lock.
>> + *
>> + * If a task holds cpu_hotplug_lock and cpuset_mutex, it blocks others,
>> + * ensuring that it is the only task able to also acquire callback_lock and
>> + * be able to modify cpusets.  It can perform various checks on the cpuset
>> + * structure first, knowing nothing will change. It can also allocate memory
>> + * without holding callback_lock. While it is performing these checks, various
>> + * callback routines can briefly acquire callback_lock to query cpusets.  Once
>> + * it is ready to make the changes, it takes callback_lock, blocking everyone
>> + * else.
>> + *
>> + * Calls to the kernel memory allocator cannot be made while holding
>> + * callback_lock which is a spinlock, as the memory allocator may sleep or
>> + * call back into cpuset code and acquire callback_lock.
>>   *
>>   * Now, the task_struct fields mems_allowed and mempolicy may be changed
>>   * by other task, we use alloc_lock in the task_struct fields to protect
>> @@ -256,6 +273,7 @@ struct cpuset top_cpuset = {
>>   * cpumasks and nodemasks.
>>   */
>>  
>> +static DEFINE_MUTEX(isolcpus_update_mutex);
>>  static DEFINE_MUTEX(cpuset_mutex);
>>  
>>  /**
>> @@ -302,7 +320,7 @@ void cpuset_full_unlock(void)
>>  #ifdef CONFIG_LOCKDEP
>>  bool lockdep_is_cpuset_held(void)
>>  {
>> -	return lockdep_is_held(&cpuset_mutex);
>> +	return lockdep_is_held(&isolcpus_update_mutex);
>>  }
>>  #endif
>>  
>> @@ -1294,9 +1312,8 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>>  static void __update_isolation_cpumasks(bool twork);
>>  static void isolation_task_work_fn(struct callback_head *cb)
>>  {
>> -	cpuset_full_lock();
>> +	guard(mutex)(&isolcpus_update_mutex);
>>  	__update_isolation_cpumasks(true);
>> -	cpuset_full_lock();
>>  }
>>  
>>  /*
>> @@ -1338,8 +1355,18 @@ static void __update_isolation_cpumasks(bool twork)
>>  		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.
>> +	 */
>> +	if (!twork)
>> +		cpuset_full_unlock();
>>  	ret = housekeeping_update(isolated_cpus);
>>  	WARN_ON_ONCE(ret < 0);
>> +	if (!twork)
>> +		cpuset_full_lock();
>>  
>>  	isolated_cpus_updating = false;
>>  }
>> @@ -3196,6 +3223,7 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>>  		return -EACCES;
>>  
>>  	buf = strstrip(buf);
>> +	mutex_lock(&isolcpus_update_mutex);
>>  	cpuset_full_lock();
>>  	if (!is_cpuset_online(cs))
>>  		goto out_unlock;
>> @@ -3226,6 +3254,7 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>>  		rebuild_sched_domains_locked();
>>  out_unlock:
>>  	cpuset_full_unlock();
>> +	mutex_unlock(&isolcpus_update_mutex);
>>  	if (of_cft(of)->private == FILE_MEMLIST)
>>  		schedule_flush_migrate_mm();
>>  	return retval ?: nbytes;
>> @@ -3329,6 +3358,7 @@ static ssize_t cpuset_partition_write(struct kernfs_open_file *of, char *buf,
>>  	else
>>  		return -EINVAL;
>>  
>> +	guard(mutex)(&isolcpus_update_mutex);
>>  	cpuset_full_lock();
>>  	if (is_cpuset_online(cs))
>>  		retval = update_prstate(cs, val);
>> @@ -3502,6 +3532,7 @@ static void cpuset_css_killed(struct cgroup_subsys_state *css)
>>  {
>>  	struct cpuset *cs = css_cs(css);
>>  
>> +	guard(mutex)(&isolcpus_update_mutex);
>>  	cpuset_full_lock();
>>  	/* Reset valid partition back to member */
>>  	if (is_partition_valid(cs))
>> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
>> index 3b725d39c06e..ef152d401fe2 100644
>> --- a/kernel/sched/isolation.c
>> +++ b/kernel/sched/isolation.c
>> @@ -123,8 +123,6 @@ int housekeeping_update(struct cpumask *isol_mask)
>>  	struct cpumask *trial, *old = NULL;
>>  	int err;
>>  
>> -	lockdep_assert_cpus_held();
>> -
>>  	trial = kmalloc(cpumask_size(), GFP_KERNEL);
>>  	if (!trial)
>>  		return -ENOMEM;
>> @@ -136,7 +134,7 @@ int housekeeping_update(struct cpumask *isol_mask)
>>  	}
>>  
>>  	if (!housekeeping.flags)
>> -		static_branch_enable_cpuslocked(&housekeeping_overridden);
>> +		static_branch_enable(&housekeeping_overridden);
>>  
>>  	if (housekeeping.flags & HK_FLAG_DOMAIN)
>>  		old = housekeeping_cpumask_dereference(HK_TYPE_DOMAIN);
>> diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
>> index 6da9cd562b20..244a8d025e78 100644
>> --- a/kernel/time/timer_migration.c
>> +++ b/kernel/time/timer_migration.c
>> @@ -1559,8 +1559,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
>>  	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
>>  	int cpu;
>>  
>> -	lockdep_assert_cpus_held();
>> -
>>  	if (!works)
>>  		return -ENOMEM;
>>  	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
>> @@ -1570,6 +1568,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
>>  	 * First set previously isolated CPUs as available (unisolate).
>>  	 * This cpumask contains only CPUs that switched to available now.
>>  	 */
>> +	guard(cpus_read_lock)();
>>  	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
>>  	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
>>  
> 

-- 
Best regards,
Ridong

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