Re: [PATCH 1/3] cgroup/cpuset: Use static_branch_enable_cpuslocked() on cpusets_insane_config_key

[Waiman Long <llong@redhat.com>]

On 8/7/25 9:15 AM, Juri Lelli wrote:
> Hi,
>
> On 06/08/25 13:24, Waiman Long wrote:
>> The following lockdep splat was observed.
>>
>> [  812.359086] ============================================
>> [  812.359089] WARNING: possible recursive locking detected
>> [  812.359097] --------------------------------------------
>> [  812.359100] runtest.sh/30042 is trying to acquire lock:
>> [  812.359105] ffffffffa7f27420 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0xe/0x20
>> [  812.359131]
>> [  812.359131] but task is already holding lock:
>> [  812.359134] ffffffffa7f27420 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_write_resmask+0x98/0xa70
>>       :
>> [  812.359267] Call Trace:
>> [  812.359272]  <TASK>
>> [  812.359367]  cpus_read_lock+0x3c/0xe0
>> [  812.359382]  static_key_enable+0xe/0x20
>> [  812.359389]  check_insane_mems_config.part.0+0x11/0x30
>> [  812.359398]  cpuset_write_resmask+0x9f2/0xa70
>> [  812.359411]  cgroup_file_write+0x1c7/0x660
>> [  812.359467]  kernfs_fop_write_iter+0x358/0x530
>> [  812.359479]  vfs_write+0xabe/0x1250
>> [  812.359529]  ksys_write+0xf9/0x1d0
>> [  812.359558]  do_syscall_64+0x5f/0xe0
>>
>> Since commit d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem
>> and hotplug lock order"), the ordering of cpu hotplug lock
>> and cpuset_mutex had been reversed. That patch correctly
>> used the cpuslocked version of the static branch API to enable
>> cpusets_pre_enable_key and cpusets_enabled_key, but it didn't do the
>> same for cpusets_insane_config_key.
>>
>> The cpusets_insane_config_key can be enabled in the
>> check_insane_mems_config() which is called from update_nodemask()
>> or cpuset_hotplug_update_tasks() with both cpu hotplug lock and
>> cpuset_mutex held. Deadlock can happen with a pending hotplug event that
>> tries to acquire the cpu hotplug write lock which will block further
>> cpus_read_lock() attempt from check_insane_mems_config(). Fix that by
>> switching to use static_branch_enable_cpuslocked().
>>
>> Fixes: d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem and hotplug lock order")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
> Looks good to me. Thanks for spotting and fixing this.
>
> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

It is really a corner case that is not easy to trigger. I would have 
missed that myself.

Thanks,
Longman

>
> Best,
> Juri
>