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

From: chenridong
Date: Wed Jul 31 2024 - 22:14:26 EST



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@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(-)

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?