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

[Michal Koutný]

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.)

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.


> 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@xxxxxxxxxx/T/#t
> Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
> ---
>  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@xxxxxxxx>

Regards,
Michal

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