Re: regression 4.4: deadlock in with cgroup percpu_rwsem

[Christian Borntraeger]

On 01/14/2016 12:19 PM, Christian Borntraeger wrote:
> Folks,


FWIW, it _LOOKS_ like it was introduced between 4.4-rc4 and 4.4-rc5


> 
> With 4.4 I can easily bring the system into a hang like situation by
> putting stress on the cgroup_threadgroup rwsem. (e.g. starting/stopping
> kvm guests via libvirt and many vCPUs). Here is my preliminary analysis:
> 
> When the hang happens, the system is idle for all CPUs. There are some
> processes waiting for the cgroup_thread_rwsem, e.g.
> 
> crash> bt 87399
> PID: 87399  TASK: faef084998        CPU: 59  COMMAND: "systemd-udevd"
>  #0 [f9e762fc88] __schedule at 83b2cc
>  #1 [f9e762fcf0] schedule at 83ba26
>  #2 [f9e762fd08] rwsem_down_read_failed at 83fb64
>  #3 [f9e762fd68] percpu_down_read at 1bdf56
>  #4 [f9e762fdd0] exit_signals at 1742ae
>  #5 [f9e762fe00] do_exit at 163be0
>  #6 [f9e762fe60] do_group_exit at 165c62
>  #7 [f9e762fe90] __wake_up_parent at 165d00
>  #8 [f9e762fea8] system_call at 842386
> 
> of course, any new process would wait for the same lock during fork.
> 
> Looking at the rwsem, while all CPUs are idle, it appears that the lock
> is taken for write:
> 
> crash> print /x cgroup_threadgroup_rwsem.rw_sem
> $8 = {
>   count = 0xfffffffe00000001, 
> [..]
>   owner = 0xfabf28c998, 
> }
> 
> Looking at the owner field:
> 
> crash> bt 0xfabf28c998
> PID: 11867  TASK: fabf28c998        CPU: 42  COMMAND: "libvirtd"
>  #0 [fadeccb5e8] __schedule at 83b2cc
>  #1 [fadeccb650] schedule at 83ba26
>  #2 [fadeccb668] schedule_timeout at 8403c6
>  #3 [fadeccb748] wait_for_common at 83c850
>  #4 [fadeccb7b8] flush_work at 18064a
>  #5 [fadeccb8d8] lru_add_drain_all at 2abd10
>  #6 [fadeccb938] migrate_prep at 309ed2
>  #7 [fadeccb950] do_migrate_pages at 2f7644
>  #8 [fadeccb9f0] cpuset_migrate_mm at 220848
>  #9 [fadeccba58] cpuset_attach at 223248
> #10 [fadeccbaa0] cgroup_taskset_migrate at 21a678
> #11 [fadeccbaf8] cgroup_migrate at 21a942
> #12 [fadeccbba0] cgroup_attach_task at 21ab8a
> #13 [fadeccbc18] __cgroup_procs_write at 21affa
> #14 [fadeccbc98] cgroup_file_write at 216be0
> #15 [fadeccbd08] kernfs_fop_write at 3aa088
> #16 [fadeccbd50] __vfs_write at 319782
> #17 [fadeccbe08] vfs_write at 31a1ac
> #18 [fadeccbe68] sys_write at 31af06
> #19 [fadeccbea8] system_call at 842386
>  PSW:  0705100180000000 000003ff9438f9f0 (user space)
> 
> it appears that the write holder scheduled away and waits
> for a completion. Now what happens is, that the write lock
> holder finally calls flush_work for the lru_add_drain_all
> work. 
> 
> As far as I can see, this work is now tries to create a new kthread
> and waits for that, as the backtrace for the kworker on that cpu has:
> 
> PID: 81913  TASK: fab5356220        CPU: 42  COMMAND: "kworker/42:2"
>  #0 [fadd6d7998] __schedule at 83b2cc
>  #1 [fadd6d7a00] schedule at 83ba26
>  #2 [fadd6d7a18] schedule_timeout at 8403c6
>  #3 [fadd6d7af8] wait_for_common at 83c850
>  #4 [fadd6d7b68] wait_for_completion_killable at 83c996
>  #5 [fadd6d7b88] kthread_create_on_node at 1876a4
>  #6 [fadd6d7cc0] create_worker at 17d7fa
>  #7 [fadd6d7d30] worker_thread at 17fff0
>  #8 [fadd6d7da0] kthread at 187884
>  #9 [fadd6d7ea8] kernel_thread_starter at 842552
> 
> Problem is that kthreadd then needs the cgroup lock for reading,
> while libvirtd still has the lock for writing.
> 
> crash> bt 0xfaf031e220
> PID: 2      TASK: faf031e220        CPU: 40  COMMAND: "kthreadd"
>  #0 [faf034bad8] __schedule at 83b2cc
>  #1 [faf034bb40] schedule at 83ba26
>  #2 [faf034bb58] rwsem_down_read_failed at 83fb64
>  #3 [faf034bbb8] percpu_down_read at 1bdf56
>  #4 [faf034bc20] copy_process at 15eab6
>  #5 [faf034bd08] _do_fork at 160430
>  #6 [faf034bdd0] kernel_thread at 160a82
>  #7 [faf034be30] kthreadd at 188580
>  #8 [faf034bea8] kernel_thread_starter at 842552
> 
> BANG.kthreadd waits for the lock that libvirtd hold, and libvirtd waits
> for kthreadd to finish some task
> 
> Reverting 001dac627ff374 ("locking/percpu-rwsem: Make use of the rcu_sync 
> infrastructure") does not help, so it does not seem to be related to the
> rcu_sync rework.
> 
> Any ideas, questions (dump is still available)
> 
> PS: not sure if lockdep could detect such a situation. it is running but silent.
> 
> 
> Christian
>