On 2024/9/9 22:19, Michal Koutný wrote:
On Sat, Aug 17, 2024 at 09:33:34AM GMT, Chen Ridong <chenridong@xxxxxxxxxx> 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@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(-)
I have convinved myself now that you can put
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>
Regards,
Michal
Thank you very much.
Best Regards,
Ridong
