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Date:   Fri, 20 Nov 2020 10:26:13 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Marco Elver <elver@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kasan-dev <kasan-dev@...glegroups.com>, rcu@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH]
 kfence: Avoid stalling...)

On Fri, 20 Nov 2020 15:19:28 +0100
Marco Elver <elver@...gle.com> wrote:

> None of those triggered either.
> 
> I found that disabling ftrace for some of kernel/rcu (see below) solved
> the stalls (and any mention of deadlocks as a side-effect I assume),
> resulting in successful boot.
> 
> Does that provide any additional clues? I tried to narrow it down to 1-2
> files, but that doesn't seem to work.
> 
> Thanks,
> -- Marco
> 
> ------ >8 ------  
> 
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 0cfb009a99b9..678b4b094f94 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -3,6 +3,13 @@
>  # and is generally not a function of system call inputs.
>  KCOV_INSTRUMENT := n
>  
> +ifdef CONFIG_FUNCTION_TRACER
> +CFLAGS_REMOVE_update.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_sync.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_srcutree.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_tree.o = $(CC_FLAGS_FTRACE)
> +endif
> +

Can you narrow it down further? That is, do you really need all of the
above to stop the stalls?

Also, since you are using linux-next, you have ftrace recursion debugging.
Please enable:

CONFIG_FTRACE_RECORD_RECURSION=y
CONFIG_RING_BUFFER_RECORD_RECURSION=y

when enabling any of the above. If you can get to a successful boot, you
can then:

 # cat /sys/kernel/tracing/recursed_functions

Which would let me know if there's an recursion issue in RCU somewhere.

-- Steve


-- Steve

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