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

On 2024/8/22 8:57, Chen Ridong wrote:

On 2024/8/17 17:33, Chen Ridong wrote:

quoted

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 (local)
Signed-off-by: Chen Ridong <redacted>
---
  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

Friendly ping.