On 3/2/26 7:14 AM, Frederic Weisbecker wrote:
On Sat, Feb 21, 2026 at 01:54:18PM -0500, 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
Which workqueue is involved here that holds rtnl_mutex?
Is this an existing problem or added test code?
Circular locking dependency here may not necessarily mean that
rtnl_mutex is directly used in a work function. However it can be used
in a locking chain involving multiple parties that can result in a
deadlock situation if they happen in the right order. So it is better
safe that sorry even if the chance of this occurrence is minimal.
Cheers,
Longman
