Re: [PATCH] ftrace: Fix missing synchronize_rcu() removing trampoline from kallsyms

[peterz]

On Tue, Sep 01, 2020 at 12:16:17PM +0300, Adrian Hunter wrote:
> Add synchronize_rcu() after list_del_rcu() in
> ftrace_remove_trampoline_from_kallsyms() to protect readers of
> ftrace_ops_trampoline_list (in ftrace_get_trampoline_kallsym)
> which is used when kallsyms is read.
> 
> Fixes: fc0ea795f53c8d ("ftrace: Add symbols for ftrace trampolines")
> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> ---
>  kernel/trace/ftrace.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 275441254bb5..4e64367c9774 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2782,6 +2782,7 @@ static void ftrace_remove_trampoline_from_kallsyms(struct ftrace_ops *ops)
>  {
>  	lockdep_assert_held(&ftrace_lock);
>  	list_del_rcu(&ops->list);
> +	synchronize_rcu();
>  }


Hurmph, we've just done a ton of that:


	ftrace_shutdown()
	  synchronize_rcu_tasks_rude()
	  ftrace_trampoline_free()
	    ftrace_remove_trampoline_from_kallsyms()


So would it not be better to move that call before the existing
synchronize_rcu_tasks stuff rather than adding another synchronize_rcu()
call?