Re: [PATCH] RISC-V: Don't check text_mutex during stop_machine

[Steven Rostedt]

On Thu,  6 May 2021 00:10:41 -0700
Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:

> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> index 7f1e5203de88..da2405652f1d 100644
> --- a/arch/riscv/kernel/ftrace.c
> +++ b/arch/riscv/kernel/ftrace.c
> @@ -11,6 +11,8 @@
>  #include <asm/cacheflush.h>
>  #include <asm/patch.h>
>  
> +int riscv_ftrace_in_stop_machine;
> +
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
>  {
> @@ -232,3 +234,16 @@ int ftrace_disable_ftrace_graph_caller(void)
>  }
>  #endif /* CONFIG_DYNAMIC_FTRACE */
>  #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> +
> +void arch_ftrace_update_code(int command)
> +{
> +	/*
> +	 * The code sequences we use for ftrace can't be patched while the
> +	 * kernel is running, so we need to use stop_machine() to modify them
> +	 * for now.  This doesn't play nice with text_mutex, we use this flag
> +	 * to elide the check.
> +	 */
> +	riscv_ftrace_in_stop_machine = true;
> +	ftrace_run_stop_machine(command);
> +	riscv_ftrace_in_stop_machine = false;
> +}
> diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> index 0b552873a577..7983dba477f0 100644

This would work, but my suggestion was to do it without having to add this
arch function. Because the caller of this is:

static void ftrace_run_update_code(int command)
{
	int ret;

	ret = ftrace_arch_code_modify_prepare();
	FTRACE_WARN_ON(ret);
	if (ret)
		return;

	/*
	 * By default we use stop_machine() to modify the code.
	 * But archs can do what ever they want as long as it
	 * is safe. The stop_machine() is the safest, but also
	 * produces the most overhead.
	 */
	arch_ftrace_update_code(command);

	ret = ftrace_arch_code_modify_post_process();
	FTRACE_WARN_ON(ret);
}


Where you already have two hooks that you use to take the text_mutex before
calling arch_ftrace_update_code().

In RISC-V those are:

int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
{
        mutex_lock(&text_mutex);
        return 0;
}

int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
{
        mutex_unlock(&text_mutex);
        return 0;
}

Where all you have to do is change them to:

int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
{
        mutex_lock(&text_mutex);
	riscv_ftrace_in_stop_machine = true;
        return 0;
}

int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
{
	riscv_ftrace_in_stop_machine = false;
        mutex_unlock(&text_mutex);
        return 0;
}

And you have the exact same affect. Those functions are only used before
calling the stop machine code you have.

-- Steve