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@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. 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!
