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    SubjectRE: kdump always hangs in rcu_barrier() -> wait_for_completion()
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    > From: Paul E. McKenney <paulmck@kernel.org>
    > Sent: Thursday, November 26, 2020 7:47 AM
    > ...
    > The rcu_segcblist_n_cbs() function returns non-zero because something
    > invoked call_rcu() some time previously. The ftrace facility (or just
    > a printk) should help you work out where that call_rcu() is located.

    call_rcu() is indeed called multiple times, but as you said, this should
    be normal.

    > My best guess is that the underlying bug is that you are invoking
    > rcu_barrier() before the RCU grace-period kthread has been created.
    > This means that RCU grace periods cannot complete, which in turn means
    > that if there has been even one invocation of call_rcu() since boot,
    > rcu_barrier() cannot complete, which is what you are in fact seeing.
    > Please note that it is perfectly legal to invoke call_rcu() very early in
    > the boot process, as in even before the call to rcu_init(). Therefore,
    > if this is the case, the bug is the early call to rcu_barrier(), not
    > the early calls to call_rcu().
    >
    > To check this, at the beginning of rcu_barrier(), check the value of
    > rcu_state.gp_kthread. If my guess is correct, it will be NULL.

    Unluckily, it's not NULL here. :-)

    >
    > Another possibility is that rcu_state.gp_kthread is non-NULL, but that
    > something else is preventing RCU grace periods from completing, but in

    It looks like somehow the scheduling is not working here: in rcu_barrier()
    , if I replace the wait_for_completion() with
    wait_for_completion_timeout(&rcu_state.barrier_completion, 30*HZ), the
    issue persists.

    > that case you should see RCU CPU stall warnings. Unless of course they
    > have been disabled.
    > Thanx, Paul

    I guess I didn't disable the wanrings (I don't even know how to do that :)

    grep RCU .config
    # RCU Subsystem
    CONFIG_TREE_RCU=y
    # CONFIG_RCU_EXPERT is not set
    CONFIG_SRCU=y
    CONFIG_TREE_SRCU=y
    CONFIG_TASKS_RCU_GENERIC=y
    CONFIG_TASKS_RUDE_RCU=y
    CONFIG_TASKS_TRACE_RCU=y
    CONFIG_RCU_STALL_COMMON=y
    CONFIG_RCU_NEED_SEGCBLIST=y
    CONFIG_RCU_NOCB_CPU=y
    # end of RCU Subsystem
    CONFIG_MMU_GATHER_RCU_TABLE_FREE=y
    # RCU Debugging
    # CONFIG_RCU_SCALE_TEST is not set
    # CONFIG_RCU_TORTURE_TEST is not set
    # CONFIG_RCU_REF_SCALE_TEST is not set
    CONFIG_RCU_CPU_STALL_TIMEOUT=30
    CONFIG_RCU_TRACE=y
    CONFIG_RCU_EQS_DEBUG=y
    # end of RCU Debugging

    Thanks,
    -- Dexuan

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