On 1/29/26 7:56 PM, Chen Ridong wrote:
On 2026/1/30 5:16, Waiman Long wrote:
On 1/29/26 3:01 AM, 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().
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?
Hi, Longman,
In this patch, I noticed that cpuset_css_offline and cpuset_css_free do not
acquire the isolcpus_update_mutex. This could potentially lead to a UAF issue.
That is the reason why I add a new top-level isolcpus_update_mutex.
cpuset_css_killed() and the update_isolation_cpumasks()'s unlock/lock sequence
will have to acquire this isolcpus_update_mutex first.
However, simply adding isolcpus_update_mutex to cpuset_css_killed and
update_isolation_cpumasks may not be sufficient.
As I mentioned, the path that calls __update_isolation_cpumasks may first
acquire isolcpus_update_mutex and cpuset_full_lock, but once cpuset_css_killed
is completed, it will release the “full” lock and then attempt to reacquire it
later. During this intermediate period, the cpuset may have already been freed,
because cpuset_css_offline and cpuset_css_free do not currently acquire the
isolcpus_update_mutex.
You are right that acquisition of the new isolcpus_update_mutex should
be in all the places where cpuset_full_lock() is acquired. Will update
the patch to do that. That should eliminate the risk.
Cheers,
Longman
