Re: [PATCH v3 1/1] cgroup: fix deadlock caused by cgroup_mutex and cpu_hotplug_lock

[Hillf Danton]

On Mon, 9 Sep 2024 16:19:38 +0200 Michal Koutny <mkoutny@xxxxxxxx>
> On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@xxxxxxxxxx> wrote:
> > 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 cpusets are deleted asynchronously, which puts a
> >    large number of cgroup_bpf_release works into system_wq. The max_active
> >    of system_wq is WQ_DFL_ACTIVE(256). Consequently, all active works are
> >    cgroup_bpf_release works, and many cgroup_bpf_release works will be put
> >    into inactive queue. As illustrated in the diagram, there are 256 (in
> >    the acvtive queue) + n (in the inactive queue) works.
> > 2. Setting watchdog_thresh will hold cpu_hotplug_lock.read and put
> >    smp_call_on_cpu work into system_wq. However step 1 has already filled
> >    system_wq, 'sscs.work' is put into inactive queue. 'sscs.work' has
> >    to wait until the works that were put into the inacvtive queue earlier
> >    have executed (n cgroup_bpf_release), so it will be blocked for a while.
> > 3. Cpu offline requires cpu_hotplug_lock.write, which is blocked by step 2.
> > 4. Cpusets that were deleted at step 1 put cgroup_release works into
> >    cgroup_destroy_wq. They are competing to get cgroup_mutex all the time.
> >    When cgroup_metux is acqured by work at css_killed_work_fn, it will
> >    call cpuset_css_offline, which needs to acqure cpu_hotplug_lock.read.
> >    However, cpuset_css_offline will be blocked for step 3.
> > 5. At this moment, there are 256 works in active queue that are
> >    cgroup_bpf_release, they are attempting to acquire cgroup_mutex, and as
> >    a result, all of them are blocked. Consequently, sscs.work can not be
> >    executed. Ultimately, this situation leads to four processes being
> >    blocked, forming a deadlock.
> >
> > system_wq(step1)		WatchDog(step2)			cpu offline(step3)	cgroup_destroy_wq(step4)
> > ...
> > 2000+ cgroups deleted asyn
> > 256 actives + n inactives
> > 				__lockup_detector_reconfigure
> > 				P(cpu_hotplug_lock.read)
> > 				put sscs.work into system_wq
> > 256 + n + 1(sscs.work)
> > sscs.work wait to be executed
> > 				warting sscs.work finish
> > 								percpu_down_write
> > 								P(cpu_hotplug_lock.write)
> > 								...blocking...
> > 											css_killed_work_fn
> > 											P(cgroup_mutex)
> > 											cpuset_css_offline
> > 											P(cpu_hotplug_lock.read)
> > 											...blocking...
> > 256 cgroup_bpf_release
> > mutex_lock(&cgroup_mutex);
> > ..blocking...
> 
> Thanks, Ridong, for laying this out.
> Let me try to extract the core of the deps above.
> 
> The correct lock ordering is: cgroup_mutex then cpu_hotplug_lock.
> However, the smp_call_on_cpu() under cpus_read_lock may lead to
> a deadlock (ABBA over those two locks).
> 
> This is OK
> 	thread T					system_wq worker
> 
> 	  						lock(cgroup_mutex) (II)
> 							...
> 							unlock(cgroup_mutex)
> 	down(cpu_hotplug_lock.read)
> 	smp_call_on_cpu
> 	  queue_work_on(cpu, system_wq, scss) (I)
> 							scss.func
> 	  wait_for_completion(scss)
> 	up(cpu_hotplug_lock.read)
> 
> However, there is no ordering between (I) and (II) so they can also happen
> in opposite
> 
> 	thread T					system_wq worker
> 
> 	down(cpu_hotplug_lock.read)
> 	smp_call_on_cpu
> 	  queue_work_on(cpu, system_wq, scss) (I)
> 	  						lock(cgroup_mutex)  (II)
> 							...
> 							unlock(cgroup_mutex)
> 							scss.func
> 	  wait_for_completion(scss)
> 	up(cpu_hotplug_lock.read)
> 
> And here the thread T + system_wq worker effectively call
> cpu_hotplug_lock and cgroup_mutex in the wrong order. (And since they're
> two threads, it won't be caught by lockdep.)
> 
Given no workqueue work executed without being dequeued, any queued work,
regardless if they are more than 2048, that acquires cgroup_mutex could not
prevent the work queued by thread-T from being executed, so thread-T can
make safe forward progress, therefore with no chance left for the ABBA 
deadlock you spotted where lockdep fails to work.