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

[chenridong]

On 2024/7/24 19:08, Hillf Danton wrote:
> 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.

Hello Hillf, did you mean to say this issue couldn't happen?
I wish it hadn't happen, as it took me a long time to figure out.
However, it did happen. It could be reproduced with the method I 
offered, You can access the scripts using this link: 
https://lore.kernel.org/cgroups/e90c32d2-2a85-4f28-9154-09c7d320cb60@xxxxxxxxxx/T/#t.

It's not about how the pool's workers were created, but rather the 
limit(system_wq ) of workqueue. If system_wq reaches its max_active 
limit, the work enqueued afterward will be placed on 
pwq->inactive_works. In this scenario described above, the problem is 
all active works(cgroup_bpf_release) are blocked, and the 
inactive_works(smp_call_on_cpu) couldn't be executed when it forms a loop.

We have discussed this before in V1, you can find in the Link.

Thanks
Ridong