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Message-ID: <53ed023b-c86c-498a-b1fc-2b442059f6af@huawei.com>
Date: Thu, 25 Jul 2024 09:48:36 +0800
From: chenridong <chenridong@...wei.com>
To: Hillf Danton <hdanton@...a.com>
CC: Roman Gushchin <roman.gushchin@...ux.dev>, <tj@...nel.org>,
<bpf@...r.kernel.org>, <cgroups@...r.kernel.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2] cgroup: fix deadlock caused by cgroup_mutex and
cpu_hotplug_lock
On 2024/7/24 19:08, Hillf Danton wrote:
> On Fri, 19 Jul 2024 02:52:32 +0000 Chen Ridong <chenridong@...wei.com>
>> We found a hung_task problem as shown below:
>>
>> INFO: task kworker/0:0:8 blocked for more than 327 seconds.
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:kworker/0:0 state:D stack:13920 pid:8 ppid:2 flags:0x00004000
>> Workqueue: events cgroup_bpf_release
>> Call Trace:
>> <TASK>
>> __schedule+0x5a2/0x2050
>> ? find_held_lock+0x33/0x100
>> ? wq_worker_sleeping+0x9e/0xe0
>> schedule+0x9f/0x180
>> schedule_preempt_disabled+0x25/0x50
>> __mutex_lock+0x512/0x740
>> ? cgroup_bpf_release+0x1e/0x4d0
>> ? cgroup_bpf_release+0xcf/0x4d0
>> ? process_scheduled_works+0x161/0x8a0
>> ? cgroup_bpf_release+0x1e/0x4d0
>> ? mutex_lock_nested+0x2b/0x40
>> ? __pfx_delay_tsc+0x10/0x10
>> mutex_lock_nested+0x2b/0x40
>> cgroup_bpf_release+0xcf/0x4d0
>> ? process_scheduled_works+0x161/0x8a0
>> ? trace_event_raw_event_workqueue_execute_start+0x64/0xd0
>> ? process_scheduled_works+0x161/0x8a0
>> process_scheduled_works+0x23a/0x8a0
>> worker_thread+0x231/0x5b0
>> ? __pfx_worker_thread+0x10/0x10
>> kthread+0x14d/0x1c0
>> ? __pfx_kthread+0x10/0x10
>> ret_from_fork+0x59/0x70
>> ? __pfx_kthread+0x10/0x10
>> ret_from_fork_asm+0x1b/0x30
>> </TASK>
>>
>> This issue can be reproduced by the following methods:
>> 1. A large number of cpuset cgroups are deleted.
>> 2. Set cpu on and off repeatly.
>> 3. Set watchdog_thresh repeatly.
>>
>> 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 cgroups are deleted, which will put a large
>> number of cgroup_bpf_release works into system_wq. The max_active
>> of system_wq is WQ_DFL_ACTIVE(256). When cgroup_bpf_release can not
>> get cgroup_metux, it may cram system_wq, and it will block work
>> enqueued later.
>> 2. Setting watchdog_thresh will hold cpu_hotplug_lock.read and put
>> smp_call_on_cpu work into system_wq. However it may be blocked by
>> step 1.
>> 3. Cpu offline requires cpu_hotplug_lock.write, which is blocked by step 2.
>> 4. When a cpuset is deleted, cgroup release work is placed on
>> cgroup_destroy_wq, it will hold cgroup_metux and acquire
>> cpu_hotplug_lock.read. Acquiring cpu_hotplug_lock.read is blocked by
>> cpu_hotplug_lock.write as mentioned by step 3. Finally, it forms a
>> loop and leads to a deadlock.
>>
>> cgroup_destroy_wq(step4) cpu offline(step3) WatchDog(step2) system_wq(step1)
>> ......
>> __lockup_detector_reconfigure:
>> P(cpu_hotplug_lock.read)
>> ...
>> ...
>> percpu_down_write:
>> P(cpu_hotplug_lock.write)
>> ...256+ works
>> cgroup_bpf_release:
>> P(cgroup_mutex)
>> smp_call_on_cpu:
>> Wait system_wq
>> ...
>> css_killed_work_fn:
>> P(cgroup_mutex)
>> ...
>> cpuset_css_offline:
>> P(cpu_hotplug_lock.read)
>>
> worker_thread()
> manage_workers()
> maybe_create_worker()
> create_worker() // has nothing to do with WQ_DFL_ACTIVE
> process_scheduled_works()
>
> Given idle worker created independent of WQ_DFL_ACTIVE before handling
> work item, no deadlock could rise in your scenario above.
Hello Hillf, did you mean to say this issue couldn't happen?
I wish it hadn't happen, as it took me a long time to figure out.
However, it did happen. It could be reproduced with the method I
offered, You can access the scripts using this link:
https://lore.kernel.org/cgroups/e90c32d2-2a85-4f28-9154-09c7d320cb60@huawei.com/T/#t.
It's not about how the pool's workers were created, but rather the
limit(system_wq ) of workqueue. If system_wq reaches its max_active
limit, the work enqueued afterward will be placed on
pwq->inactive_works. In this scenario described above, the problem is
all active works(cgroup_bpf_release) are blocked, and the
inactive_works(smp_call_on_cpu) couldn't be executed when it forms a loop.
We have discussed this before in V1, you can find in the Link.
Thanks
Ridong
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