Re: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...)

Steven Rostedt
2020-11-20T15:26:54Z

On Fri, 20 Nov 2020 15:19:28 +0100
Marco Elver <elver@google.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