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Date:   Thu, 26 Nov 2020 21:25:28 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "boqun.feng@...il.com" <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: kdump always hangs in rcu_barrier() -> wait_for_completion()

> From: Paul E. McKenney <paulmck@...nel.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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