[PATCH v3 0/1] Fix deadlock caused by cgroup_mutex and cpu_hotplug_lock

[PATCH v3 0/1] Fix deadlock caused by cgroup_mutex and cpu_hotplug_lock

[Chen Ridong]

changes from v1/v2:
- Optimize commit msg.

Link v1: https://lore.kernel.org/cgroups/20240607110313.2230669-1-chenridong@huawei.com/
Link v2: https://lore.kernel.org/cgroups/20240719025232.2143638-1-chenridong@huawei.com/

Chen Ridong (1):
  cgroup: fix deadlock caused by cgroup_mutex and cpu_hotplug_lock

 kernel/bpf/cgroup.c             | 2 +-
 kernel/cgroup/cgroup-internal.h | 1 +
 kernel/cgroup/cgroup.c          | 2 +-
 3 files changed, 3 insertions(+), 2 deletions(-)

-- 
2.34.1

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

[Chen Ridong]

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,
-- 
2.34.1

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

[Chen Ridong]

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

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

[Chen Ridong]

On 2024/8/22 8:57, Chen Ridong wrote:
> 
> 
> 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
> 
Friendly ping.

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

[Michal Koutný]

[-- Attachment #1: Type: text/plain, Size: 5924 bytes --]

On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@huawei.com> wrote:
> 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...

Thanks, Ridong, for laying this out.
Let me try to extract the core of the deps above.

The correct lock ordering is: cgroup_mutex then cpu_hotplug_lock.
However, the smp_call_on_cpu() under cpus_read_lock may lead to
a deadlock (ABBA over those two locks).

This is OK
	thread T					system_wq worker
	
	  						lock(cgroup_mutex) (II)
							...
							unlock(cgroup_mutex)
	down(cpu_hotplug_lock.read)
	smp_call_on_cpu
	  queue_work_on(cpu, system_wq, scss) (I)
							scss.func
	  wait_for_completion(scss)
	up(cpu_hotplug_lock.read)

However, there is no ordering between (I) and (II) so they can also happen
in opposite

	thread T					system_wq worker
	
	down(cpu_hotplug_lock.read)
	smp_call_on_cpu
	  queue_work_on(cpu, system_wq, scss) (I)
	  						lock(cgroup_mutex)  (II)
							...
							unlock(cgroup_mutex)
							scss.func
	  wait_for_completion(scss)
	up(cpu_hotplug_lock.read)

And here the thread T + system_wq worker effectively call
cpu_hotplug_lock and cgroup_mutex in the wrong order. (And since they're
two threads, it won't be caught by lockdep.)

By that reasoning any holder of cgroup_mutex on system_wq makes system
susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
writers + cpuset operations). And the two work items must meet in same
worker's processing hence probability is low (zero?) with less than
WQ_DFL_ACTIVE items.

(And more generally, any lock that is ordered before cpu_hotplug_lock
should not be taken in system_wq work functions. Or at least such works
items should not saturate WQ_DFL_ACTIVE workers.)

Wrt other uses of cgroup_mutex, I only see
  bpf_map_free_in_work
    queue_work(system_unbound_wq)
      bpf_map_free_deferred
        ops->map_free == cgroup_storage_map_free
          cgroup_lock()
which is safe since it uses a different workqueue than system_wq.

> To fix the problem, place cgroup_bpf_release works on cgroup_destroy_wq,
> which can break the loop and solve the problem.

Yes, it moves the problematic cgroup_mutex holder away from system_wq
and cgroup_destroy_wq could not cause similar problems because there are
no explicit waiter for particular work items or its flushing.


> 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.

Actually, I'm not sure (because I lack workqueue knowledge) if producing
less than WQ_DFL_ACTIVE work items completely eliminates the chance of
two offending work items producing the wrong lock ordering.


> Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")

I'm now indifferent whether this is needed (perhaps in the sense it is
the _latest_ of multiple changes that contributed to possibility of this
deadlock scenario).


> 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(-)

I have convinved myself now that you can put

Reviewed-by: Michal Koutný <mkoutny@suse.com>

Regards,
Michal

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

[Chen Ridong]

On 2024/9/9 22:19, Michal Koutný wrote:
> On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@huawei.com> wrote:
>> 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...
> 
> Thanks, Ridong, for laying this out.
> Let me try to extract the core of the deps above.
> 
> The correct lock ordering is: cgroup_mutex then cpu_hotplug_lock.
> However, the smp_call_on_cpu() under cpus_read_lock may lead to
> a deadlock (ABBA over those two locks).
> 

That's right.

> This is OK
> 	thread T					system_wq worker
> 	
> 	  						lock(cgroup_mutex) (II)
> 							...
> 							unlock(cgroup_mutex)
> 	down(cpu_hotplug_lock.read)
> 	smp_call_on_cpu
> 	  queue_work_on(cpu, system_wq, scss) (I)
> 							scss.func
> 	  wait_for_completion(scss)
> 	up(cpu_hotplug_lock.read)
> 
> However, there is no ordering between (I) and (II) so they can also happen
> in opposite
> 
> 	thread T					system_wq worker
> 	
> 	down(cpu_hotplug_lock.read)
> 	smp_call_on_cpu
> 	  queue_work_on(cpu, system_wq, scss) (I)
> 	  						lock(cgroup_mutex)  (II)
> 							...
> 							unlock(cgroup_mutex)
> 							scss.func
> 	  wait_for_completion(scss)
> 	up(cpu_hotplug_lock.read)
> 
> And here the thread T + system_wq worker effectively call
> cpu_hotplug_lock and cgroup_mutex in the wrong order. (And since they're
> two threads, it won't be caught by lockdep.)
> 
> By that reasoning any holder of cgroup_mutex on system_wq makes system
> susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
> writers + cpuset operations). And the two work items must meet in same
> worker's processing hence probability is low (zero?) with less than
> WQ_DFL_ACTIVE items.
> 
> (And more generally, any lock that is ordered before cpu_hotplug_lock
> should not be taken in system_wq work functions. Or at least such works
> items should not saturate WQ_DFL_ACTIVE workers.)
> 
> Wrt other uses of cgroup_mutex, I only see
>    bpf_map_free_in_work
>      queue_work(system_unbound_wq)
>        bpf_map_free_deferred
>          ops->map_free == cgroup_storage_map_free
>            cgroup_lock()
> which is safe since it uses a different workqueue than system_wq.
> 
>> To fix the problem, place cgroup_bpf_release works on cgroup_destroy_wq,
>> which can break the loop and solve the problem.
> 
> Yes, it moves the problematic cgroup_mutex holder away from system_wq
> and cgroup_destroy_wq could not cause similar problems because there are
> no explicit waiter for particular work items or its flushing.
> 
> 
>> 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.
> 
> Actually, I'm not sure (because I lack workqueue knowledge) if producing
> less than WQ_DFL_ACTIVE work items completely eliminates the chance of
> two offending work items producing the wrong lock ordering.
> 

If producing less than WQ_DFL_ACTIVE work items, it won't lead to a 
deadlock. Because scss.func can be executed and doesn't have to wait for 
work that holds cgroup_mutex to be completed. Therefore, the probability 
is low and this issue can only be reproduced under pressure test.

> 
>> Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")
> 
> I'm now indifferent whether this is needed (perhaps in the sense it is
> the _latest_ of multiple changes that contributed to possibility of this
> deadlock scenario).
> 
> 
>> 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(-)
> 
> I have convinved myself now that you can put
> 
> Reviewed-by: Michal Koutný <mkoutny@suse.com>
> 
> Regards,
> Michal

Thank you very much.

Best Regards,
Ridong

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

[Roman Gushchin]

On Tue, Sep 10, 2024 at 09:31:41AM +0800, Chen Ridong wrote:
> 
> 
> On 2024/9/9 22:19, Michal Koutný wrote:
> > On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@huawei.com> wrote:
> > > 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...
> > 
> > Thanks, Ridong, for laying this out.
> > Let me try to extract the core of the deps above.
> > 
> > The correct lock ordering is: cgroup_mutex then cpu_hotplug_lock.
> > However, the smp_call_on_cpu() under cpus_read_lock may lead to
> > a deadlock (ABBA over those two locks).
> > 
> 
> That's right.
> 
> > This is OK
> > 	thread T					system_wq worker
> > 	
> > 	  						lock(cgroup_mutex) (II)
> > 							...
> > 							unlock(cgroup_mutex)
> > 	down(cpu_hotplug_lock.read)
> > 	smp_call_on_cpu
> > 	  queue_work_on(cpu, system_wq, scss) (I)
> > 							scss.func
> > 	  wait_for_completion(scss)
> > 	up(cpu_hotplug_lock.read)
> > 
> > However, there is no ordering between (I) and (II) so they can also happen
> > in opposite
> > 
> > 	thread T					system_wq worker
> > 	
> > 	down(cpu_hotplug_lock.read)
> > 	smp_call_on_cpu
> > 	  queue_work_on(cpu, system_wq, scss) (I)
> > 	  						lock(cgroup_mutex)  (II)
> > 							...
> > 							unlock(cgroup_mutex)
> > 							scss.func
> > 	  wait_for_completion(scss)
> > 	up(cpu_hotplug_lock.read)
> > 
> > And here the thread T + system_wq worker effectively call
> > cpu_hotplug_lock and cgroup_mutex in the wrong order. (And since they're
> > two threads, it won't be caught by lockdep.)
> > 
> > By that reasoning any holder of cgroup_mutex on system_wq makes system
> > susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
> > writers + cpuset operations). And the two work items must meet in same
> > worker's processing hence probability is low (zero?) with less than
> > WQ_DFL_ACTIVE items.

Right, I'm on the same page. Should we document then somewhere that
the cgroup mutex can't be locked from a system wq context?

I think thus will also make the Fixes tag more meaningful.

Thank you!

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

[Tejun Heo]

On Tue, Sep 10, 2024 at 09:02:59PM +0000, Roman Gushchin wrote:
...
> > > By that reasoning any holder of cgroup_mutex on system_wq makes system
> > > susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
> > > writers + cpuset operations). And the two work items must meet in same
> > > worker's processing hence probability is low (zero?) with less than
> > > WQ_DFL_ACTIVE items.
> 
> Right, I'm on the same page. Should we document then somewhere that
> the cgroup mutex can't be locked from a system wq context?
> 
> I think thus will also make the Fixes tag more meaningful.

I think that's completely fine. What's not fine is saturating system_wq.
Anything which creates a large number of concurrent work items should be
using its own workqueue. If anything, workqueue needs to add a warning for
saturation conditions and who are the offenders.

Thanks.

-- 
tejun

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

[Chen Ridong]

On 2024/9/11 5:17, Tejun Heo wrote:
> On Tue, Sep 10, 2024 at 09:02:59PM +0000, Roman Gushchin wrote:
> ...
>>>> By that reasoning any holder of cgroup_mutex on system_wq makes system
>>>> susceptible to a deadlock (in presence of cpu_hotplug_lock waiting
>>>> writers + cpuset operations). And the two work items must meet in same
>>>> worker's processing hence probability is low (zero?) with less than
>>>> WQ_DFL_ACTIVE items.
>>
>> Right, I'm on the same page. Should we document then somewhere that
>> the cgroup mutex can't be locked from a system wq context?
>>
>> I think thus will also make the Fixes tag more meaningful.
> 
> I think that's completely fine. What's not fine is saturating system_wq.
> Anything which creates a large number of concurrent work items should be
> using its own workqueue. If anything, workqueue needs to add a warning for
> saturation conditions and who are the offenders.
> 
> Thanks.
> 

I will add a patch do document that.
Should we modify WQ_DFL_ACTIVE(256 now)? Maybe 1024 is acceptable?

Best regards,
Ridong

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

[Tejun Heo]

On Thu, Sep 12, 2024 at 09:33:23AM +0800, Chen Ridong wrote:
> I will add a patch do document that.
> Should we modify WQ_DFL_ACTIVE(256 now)? Maybe 1024 is acceptable?

Yeah, let's bump it.

Thanks.

-- 
tejun