Re: [PATCH -v2] cgroup: fix deadlock caused by cgroup_mutex and cpu_hotplug_lock

[chenridong <chenridong@huawei.com>]

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@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 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.