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

[Roman Gushchin]

On Tue, Sep 10, 2024 at 09:31:41AM +0800, Chen Ridong wrote:
> 
> 
> On 2024/9/9 22:19, Michal Koutný wrote:
> > 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).
> > 
> 
> That's right.
> 
> > 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.)
> > 
> > By that reasoning any holder of cgroup_mutex on system_wq makes system
> > susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
> > writers + cpuset operations). And the two work items must meet in same
> > worker's processing hence probability is low (zero?) with less than
> > WQ_DFL_ACTIVE items.

Right, I'm on the same page. Should we document then somewhere that
the cgroup mutex can't be locked from a system wq context?

I think thus will also make the Fixes tag more meaningful.

Thank you!