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

Chen Ridong
2024-09-10T01:31:48Z

On 2024/9/9 22:19, Michal Koutný wrote:
> On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@huawei.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@huawei.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@suse.com>
>
> Regards,
> Michal

Thank you very much.

Best Regards,
Ridong