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

[Changbin Du]

On Mon, Mar 21, 2022 at 07:23:31PM -0700, Palmer Dabbelt wrote:
> From: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> 
> We're currently using stop_machine() to update ftrace, which means that
> the thread that takes text_mutex during ftrace_prepare() may not be the
> same as the thread that eventually patches the code.  This isn't
> actually a race because the lock is still held (preventing any other
> concurrent accesses) and there is only one thread running during
> stop_machine(), but it does trigger a lockdep failure.
> 
> This patch just elides the lockdep check during stop_machine.
> 
> Fixes: c15ac4fd60d5 ("riscv/ftrace: Add dynamic function tracer support")
> Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Reported-by: Changbin Du <changbin.du@xxxxxxxxx>
> Signed-off-by: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> 
> --
> 
> Changes since v1 [<20210506071041.417854-1-palmer@xxxxxxxxxxx>]:
> * Use ftrace_arch_ocde_modify_{prepare,post_process}() to set the flag.
>   I remember having a reason I wanted the function when I wrote the v1,
>   but it's been almost a year and I forget what that was -- maybe I was
>   just crazy, the patch was sent at midnight.
> * Fix DYNAMIC_FTRACE=n builds.
> ---
>  arch/riscv/include/asm/ftrace.h |  7 +++++++
>  arch/riscv/kernel/ftrace.c      | 12 ++++++++++++
>  arch/riscv/kernel/patch.c       | 10 +++++++++-
>  3 files changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> index 04dad3380041..3ac7609f4ee9 100644
> --- a/arch/riscv/include/asm/ftrace.h
> +++ b/arch/riscv/include/asm/ftrace.h
> @@ -81,8 +81,15 @@ do {									\
>  struct dyn_ftrace;
>  int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
>  #define ftrace_init_nop ftrace_init_nop
> +extern int riscv_ftrace_in_stop_machine;
>  #endif
>  
> +#else /* CONFIG_DYNAMIC_FTRACE */
> +
> +#ifndef __ASSEMBLY__
> +#define riscv_ftrace_in_stop_machine 0
>  #endif
>  
> +#endif /* CONFIG_DYNAMIC_FTRACE */
> +
>  #endif /* _ASM_RISCV_FTRACE_H */
> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> index 4716f4cdc038..c5f77922d7ea 100644
> --- a/arch/riscv/kernel/ftrace.c
> +++ b/arch/riscv/kernel/ftrace.c
> @@ -11,15 +11,27 @@
>  #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)
>  {
>  	mutex_lock(&text_mutex);
> +
> +	/*
> +	 * 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;
> +
>  	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;
>  }

The function ftrace_init_nop() should be updated. That's not in stop-machine
context.

@@ -136,9 +124,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
 {
        int out;

-       ftrace_arch_code_modify_prepare();
+       mutex_lock(&text_mutex);
        out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
-       ftrace_arch_code_modify_post_process();
+       mutex_unlock(&text_mutex);


-- 
Cheers,
Changbin Du