• Prateek Sood's avatar
    cgroup: Fix deadlock in cpu hotplug path · 35b80e75
    Prateek Sood authored
    commit 116d2f74 upstream.
    
    Deadlock during cgroup migration from cpu hotplug path when a task T is
    being moved from source to destination cgroup.
    
    kworker/0:0
    cpuset_hotplug_workfn()
       cpuset_hotplug_update_tasks()
          hotplug_update_tasks_legacy()
            remove_tasks_in_empty_cpuset()
              cgroup_transfer_tasks() // stuck in iterator loop
                cgroup_migrate()
                  cgroup_migrate_add_task()
    
    In cgroup_migrate_add_task() it checks for PF_EXITING flag of task T.
    Task T will not migrate to destination cgroup. css_task_iter_start()
    will keep pointing to task T in loop waiting for task T cg_list node
    to be removed.
    
    Task T
    do_exit()
      exit_signals() // sets PF_EXITING
      exit_task_namespaces()
        switch_task_namespaces()
          free_nsproxy()
            put_mnt_ns()
              drop_collected_mounts()
                namespace_unlock()
                  synchronize_rcu()
                    _synchronize_rcu_expedited()
                      schedule_work() // on cpu0 low priority worker pool
                      wait_event() // waiting for work item to execute
    
    Task T inserted a work item in the worklist of cpu0 low priority
    worker pool. It is waiting for expedited grace period work item
    to execute. This work item will only be executed once kworker/0:0
    complete execution of cpuset_hotplug_workfn().
    
    kworker/0:0 ==> Task T ==>kworker/0:0
    
    In case of PF_EXITING task being migrated from source to destination
    cgroup, migrate next available task in source cgroup.
    Signed-off-by: 's avatarPrateek Sood <prsood@codeaurora.org>
    Signed-off-by: 's avatarTejun Heo <tj@kernel.org>
    [AmitP: Upstream commit cherry-pick failed, so I picked the
            backported changes from CAF/msm-4.9 tree instead:
            https://source.codeaurora.org/quic/la/kernel/msm-4.9/commit/?id=49b74f1696417b270c89cd893ca9f37088928078]
    Signed-off-by: 's avatarAmit Pundir <amit.pundir@linaro.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    35b80e75