On Fri, Jan 30, 2026 at 10:42:53AM -0500, Waiman Long wrote:
> +/* Both cpuset_mutex and cpus_read_locked acquired */
> +static bool cpuset_locked;
> +
> /*
> * A flag to force sched domain rebuild at the end of an operation.
> * It can be set in
> @@ -285,10 +288,12 @@ void cpuset_full_lock(void)
> {
> cpus_read_lock();
> mutex_lock(&cpuset_mutex);
> + cpuset_locked = true;
> }
>
> void cpuset_full_unlock(void)
> {
> + cpuset_locked = false;
> mutex_unlock(&cpuset_mutex);
> cpus_read_unlock();
> }
> @@ -1293,14 +1308,30 @@ static bool prstate_housekeeping_conflict(int
> prstate, struct cpumask *new_cpus)
> */
> static void update_isolation_cpumasks(void)
> {
> - int ret;
> + static DECLARE_WORK(isolcpus_work, isolcpus_workfn);
>
> if (!isolated_cpus_updating)
> return;
>
> - ret = housekeeping_update(isolated_cpus);
> - WARN_ON_ONCE(ret < 0);
> + /*
> + * This function can be reached either directly from regular cpuset
> + * control file write (cpuset_locked) or via hotplug (cpus_write_lock
> + * && cpuset_mutex held). In the later case, we defer the
> + * housekeeping_update() call to the system_unbound_wq to avoid the
> + * possibility of deadlock. This also means that there will be a short
> + * period of time where HK_TYPE_DOMAIN housekeeping cpumask will lag
> + * behind isolated_cpus.
> + */
> + if (!cpuset_locked) {
I agree with Chen that this is bloody terrible.
At the very least this should have:
lockdep_assert_held(&cpuset_mutex);
But ideally you'd do patches against this and tip/locking/core that add
proper __guarded_by() annotations to this.
> + /*
> + * We rely on WORK_STRUCT_PENDING_BIT to not requeue a work
> + * item that is still pending.
> + */
> + queue_work(system_unbound_wq, &isolcpus_work);
> + return;
> + }
>
> + WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
> isolated_cpus_updating = false;
> }
