Re: [PATCH -v2] cgroup: fix deadlock caused by cgroup_mutex and cpu_hotplug_lock

[Hillf Danton]

On Fri, 19 Jul 2024 02:52:32 +0000 Chen Ridong <chenridong@xxxxxxxxxx>
> We found a hung_task problem as shown below:
> 
> INFO: task kworker/0:0:8 blocked for more than 327 seconds.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:kworker/0:0     state:D stack:13920 pid:8     ppid:2       flags:0x00004000
> Workqueue: events cgroup_bpf_release
> Call Trace:
>  <TASK>
>  __schedule+0x5a2/0x2050
>  ? find_held_lock+0x33/0x100
>  ? wq_worker_sleeping+0x9e/0xe0
>  schedule+0x9f/0x180
>  schedule_preempt_disabled+0x25/0x50
>  __mutex_lock+0x512/0x740
>  ? cgroup_bpf_release+0x1e/0x4d0
>  ? cgroup_bpf_release+0xcf/0x4d0
>  ? process_scheduled_works+0x161/0x8a0
>  ? cgroup_bpf_release+0x1e/0x4d0
>  ? mutex_lock_nested+0x2b/0x40
>  ? __pfx_delay_tsc+0x10/0x10
>  mutex_lock_nested+0x2b/0x40
>  cgroup_bpf_release+0xcf/0x4d0
>  ? process_scheduled_works+0x161/0x8a0
>  ? trace_event_raw_event_workqueue_execute_start+0x64/0xd0
>  ? process_scheduled_works+0x161/0x8a0
>  process_scheduled_works+0x23a/0x8a0
>  worker_thread+0x231/0x5b0
>  ? __pfx_worker_thread+0x10/0x10
>  kthread+0x14d/0x1c0
>  ? __pfx_kthread+0x10/0x10
>  ret_from_fork+0x59/0x70
>  ? __pfx_kthread+0x10/0x10
>  ret_from_fork_asm+0x1b/0x30
>  </TASK>
> 
> This issue can be reproduced by the following methods:
> 1. A large number of cpuset cgroups are deleted.
> 2. Set cpu on and off repeatly.
> 3. Set watchdog_thresh repeatly.
> 
> The reason for this issue is cgroup_mutex and cpu_hotplug_lock are
> acquired in different tasks, which may lead to deadlock.
> It can lead to a deadlock through the following steps:
> 1. A large number of cgroups are deleted, which will put a large
>    number of cgroup_bpf_release works into system_wq. The max_active
>    of system_wq is WQ_DFL_ACTIVE(256). When cgroup_bpf_release can not
>    get cgroup_metux, it may cram system_wq, and it will block work
>    enqueued later.
> 2. Setting watchdog_thresh will hold cpu_hotplug_lock.read and put
>    smp_call_on_cpu work into system_wq. However it may be blocked by
>    step 1.
> 3. Cpu offline requires cpu_hotplug_lock.write, which is blocked by step 2.
> 4. When a cpuset is deleted, cgroup release work is placed on
>    cgroup_destroy_wq, it will hold cgroup_metux and acquire
>    cpu_hotplug_lock.read. Acquiring cpu_hotplug_lock.read is blocked by
>    cpu_hotplug_lock.write as mentioned by step 3. Finally, it forms a
>    loop and leads to a deadlock.
> 
> cgroup_destroy_wq(step4)	cpu offline(step3)		WatchDog(step2)			system_wq(step1)
> 												......
> 								__lockup_detector_reconfigure:
> 								P(cpu_hotplug_lock.read)
> 								...
> 				...
> 				percpu_down_write:
> 				P(cpu_hotplug_lock.write)
> 												...256+ works
> 												cgroup_bpf_release:
> 												P(cgroup_mutex)
> 								smp_call_on_cpu:
> 								Wait system_wq
> ...
> css_killed_work_fn:
> P(cgroup_mutex)
> ...
> cpuset_css_offline:
> P(cpu_hotplug_lock.read) 
>
	worker_thread()
	manage_workers()
	maybe_create_worker()
	create_worker() // has nothing to do with WQ_DFL_ACTIVE
	process_scheduled_works()

Given idle worker created independent of WQ_DFL_ACTIVE before handling
work item, no deadlock could rise in your scenario above.