Message-ID: <07501c67-3b18-48e3-8929-e773d8d6920f@huaweicloud.com>
Date: Tue, 10 Sep 2024 09:31:41 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Michal Koutný <mkoutny@...e.com>,
 Chen Ridong <chenridong@...wei.com>
Cc: martin.lau@...ux.dev, ast@...nel.org, daniel@...earbox.net,
 andrii@...nel.org, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, tj@...nel.org,
 lizefan.x@...edance.com, hannes@...xchg.org, roman.gushchin@...ux.dev,
 bpf@...r.kernel.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] cgroup: fix deadlock caused by cgroup_mutex and
 cpu_hotplug_lock



On 2024/9/9 22:19, Michal Koutný wrote:
> On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@...wei.com> 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.
> 
> (And more generally, any lock that is ordered before cpu_hotplug_lock
> should not be taken in system_wq work functions. Or at least such works
> items should not saturate WQ_DFL_ACTIVE workers.)
> 
> Wrt other uses of cgroup_mutex, I only see
>    bpf_map_free_in_work
>      queue_work(system_unbound_wq)
>        bpf_map_free_deferred
>          ops->map_free == cgroup_storage_map_free
>            cgroup_lock()
> which is safe since it uses a different workqueue than system_wq.
> 
>> To fix the problem, place cgroup_bpf_release works on cgroup_destroy_wq,
>> which can break the loop and solve the problem.
> 
> Yes, it moves the problematic cgroup_mutex holder away from system_wq
> and cgroup_destroy_wq could not cause similar problems because there are
> no explicit waiter for particular work items or its flushing.
> 
> 
>> System wqs are for misc things which shouldn't create a large number
>> of concurrent work items.  If something is going to generate
>>> WQ_DFL_ACTIVE(256) concurrent work
>> items, it should use its own dedicated workqueue.
> 
> Actually, I'm not sure (because I lack workqueue knowledge) if producing
> less than WQ_DFL_ACTIVE work items completely eliminates the chance of
> two offending work items producing the wrong lock ordering.
> 

If producing less than WQ_DFL_ACTIVE work items, it won't lead to a 
deadlock. Because scss.func can be executed and doesn't have to wait for 
work that holds cgroup_mutex to be completed. Therefore, the probability 
is low and this issue can only be reproduced under pressure test.

> 
>> Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")
> 
> I'm now indifferent whether this is needed (perhaps in the sense it is
> the _latest_ of multiple changes that contributed to possibility of this
> deadlock scenario).
> 
> 
>> Link: https://lore.kernel.org/cgroups/e90c32d2-2a85-4f28-9154-09c7d320cb60@huawei.com/T/#t
>> Signed-off-by: Chen Ridong <chenridong@...wei.com>
>> ---
>>   kernel/bpf/cgroup.c             | 2 +-
>>   kernel/cgroup/cgroup-internal.h | 1 +
>>   kernel/cgroup/cgroup.c          | 2 +-
>>   3 files changed, 3 insertions(+), 2 deletions(-)
> 
> I have convinved myself now that you can put
> 
> Reviewed-by: Michal Koutný <mkoutny@...e.com>
> 
> Regards,
> Michal

Thank you very much.

Best Regards,
Ridong

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