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

Chen Ridong
2024-08-22T00:57:53Z

On 2024/8/17 17:33, 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 pressuse test:
> 1. A large number of cpuset cgroups are deleted.
> 2. Set cpu on and off repeatly.
> 3. Set watchdog_thresh repeatly.
> The scripts can be obtained at LINK mentioned above the signature.
>
> 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...
>
> 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@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(-)
>
> 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 c964dd7ff967..17ac19bc8106 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 75058fbf4450..77fa9ed69c86 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,

Ping.
Hi,TJ, Roman and Michal, I have updated commit message, I think it can
be much clearer now, can you review it again?

Thanks,
Ridong