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Message-ID: <823a5926-4ed9-4784-b9ae-ee0d0eb8ebf8@huawei.com>
Date: Thu, 1 Aug 2024 09:34:56 +0800
From: chenridong <chenridong@...wei.com>
To: <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>,
Michal Koutný <mkoutny@...e.com>
CC: <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 8:53, chenridong wrote:
>
>
> On 2024/7/19 10:52, Chen Ridong wrote:
>> 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)
>>
>> To fix the problem, place cgroup_bpf_release works on cgroup_destroy_wq,
>> which can break the loop and solve the problem. 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.
>>
>> Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from
>> cgroup itself")
>> 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(-)
>>
>> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
>> index 8ba73042a239..a611a1274788 100644
>> --- a/kernel/bpf/cgroup.c
>> +++ b/kernel/bpf/cgroup.c
>> @@ -334,7 +334,7 @@ static void cgroup_bpf_release_fn(struct
>> percpu_ref *ref)
>> struct cgroup *cgrp = container_of(ref, struct cgroup, bpf.refcnt);
>> INIT_WORK(&cgrp->bpf.release_work, cgroup_bpf_release);
>> - queue_work(system_wq, &cgrp->bpf.release_work);
>> + queue_work(cgroup_destroy_wq, &cgrp->bpf.release_work);
>> }
>> /* Get underlying bpf_prog of bpf_prog_list entry, regardless if
>> it's through
>> diff --git a/kernel/cgroup/cgroup-internal.h
>> b/kernel/cgroup/cgroup-internal.h
>> index 520b90dd97ec..9e57f3e9316e 100644
>> --- a/kernel/cgroup/cgroup-internal.h
>> +++ b/kernel/cgroup/cgroup-internal.h
>> @@ -13,6 +13,7 @@
>> extern spinlock_t trace_cgroup_path_lock;
>> extern char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
>> extern void __init enable_debug_cgroup(void);
>> +extern struct workqueue_struct *cgroup_destroy_wq;
>> /*
>> * cgroup_path() takes a spin lock. It is good practice not to take
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index e32b6972c478..3317e03fe2fb 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -124,7 +124,7 @@ DEFINE_PERCPU_RWSEM(cgroup_threadgroup_rwsem);
>> * destruction work items don't end up filling up max_active of
>> system_wq
>> * which may lead to deadlock.
>> */
>> -static struct workqueue_struct *cgroup_destroy_wq;
>> +struct workqueue_struct *cgroup_destroy_wq;
>> /* generate an array of cgroup subsystem pointers */
>> #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
>
> Friendly ping.
>
Hi, Tejun,Roman,and Michal, do you have any opinion? Can this patch be
merged?
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