Re: [PATCH/for-next v4 3/4] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock

[Waiman Long <llong@redhat.com>]

On 2/9/26 2:12 AM, Chen Ridong wrote:
>>   		return;
>>   	}
>>   
>> -	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>> -	isolated_cpus_updating = false;
>> +	/*
>> +	 * update_isolation_cpumasks() may be called more than once in the
>> +	 * same cpuset_mutex critical section.
>> +	 */
>> +	lockdep_assert_held(&cpuset_top_mutex);
>> +	if (isolcpus_twork_queued)
>> +		return;
>> +
>> +	init_task_work(&twork_cb, isolcpus_tworkfn);
>> +	if (!task_work_add(current, &twork_cb, TWA_RESUME))
>> +		isolcpus_twork_queued = true;
>> +	else
>> +		WARN_ON_ONCE(1);	/* Current task shouldn't be exiting */
>>   }
>>   
> Timeline:
>
> user A			user B
> write isolated cpus	write isolated cpus
> isolated_cpus_update
> update_isolation_cpumasks
> task_work_add
> isolcpus_twork_queued =true
>
> // before returning userspace
> // waiting for worker
> 			isolated_cpus_update
> 			if (isolcpus_twork_queued)
> 				return // Early exit
> 			// return to userspace
>
> // workqueue finishes
> // return to userspace
>
> For User B, the isolated_cpus value appears to be set and the syscall returns
> successfully to userspace. However, because isolcpus_twork_queued was already
> true (set by User A), User B's call skipped the actual mask update
> (update_isolation_cpumasks).
> Thus, the new isolated_cpus value is not yet effective in the kernel, even
> though User B's write operation returned without error.
>
> Is this a valid issue? Should User B's write be blocked?

It is perfectly possible that isolated_cpus can be modified more than 
one time from different tasks before a work or task_work function is 
executed. When that function is invoked, isolated_cpus should contain 
changes for both. It will copy isolated_cpus to isolated_hk_cpus and 
pass it to housekeeping_update(). When the 2nd work or task_work 
function is invoked, it will see that isolated_cpus match 
isolated_hk_cpus and skip the housekeeping_update() action. There is no 
need to block user B's write as only one task can update isolated_cpus 
at any time.

Cheers,
Longman