Re: [RESEND PATCH v2 1/2] ftrace: disable preemption between ftrace_test_recursion_trylock/unlock()

[Miroslav Benes <mbenes@suse.cz>]

> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> index a9f9c57..101e1fb 100644
> --- a/include/linux/trace_recursion.h
> +++ b/include/linux/trace_recursion.h
> @@ -208,13 +208,29 @@ static __always_inline void trace_clear_recursion(int bit)
>   * Use this for ftrace callbacks. This will detect if the function
>   * tracing recursed in the same context (normal vs interrupt),
>   *
> + * The ftrace_test_recursion_trylock() will disable preemption,
> + * which is required for the variant of synchronize_rcu() that is
> + * used to allow patching functions where RCU is not watching.
> + * See klp_synchronize_transition() for more details.
> + *

I think that you misunderstood. Steven proposed to put the comment before 
ftrace_test_recursion_trylock() call site in klp_ftrace_handler().

>   * Returns: -1 if a recursion happened.
>   *           >= 0 if no recursion
>   */
>  static __always_inline int ftrace_test_recursion_trylock(unsigned long ip,
>  							 unsigned long parent_ip)
>  {
> -	return trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> +	int bit;
> +
> +	bit = trace_test_and_set_recursion(ip, parent_ip, TRACE_FTRACE_START, TRACE_FTRACE_MAX);
> +	/*
> +	 * The zero bit indicate we are nested
> +	 * in another trylock(), which means the
> +	 * preemption already disabled.
> +	 */
> +	if (bit > 0)
> +		preempt_disable_notrace();
> +
> +	return bit;
>  }

[...]

> diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> index e8029ae..6e66ccd 100644
> --- a/kernel/livepatch/patch.c
> +++ b/kernel/livepatch/patch.c
> @@ -52,11 +52,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,

Here

>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (WARN_ON_ONCE(bit < 0))
>  		return;
> -	/*
> -	 * A variant of synchronize_rcu() is used to allow patching functions
> -	 * where RCU is not watching, see klp_synchronize_transition().
> -	 */
> -	preempt_disable_notrace();
> 
>  	func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
>  				      stack_node);
> @@ -120,7 +115,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>  	klp_arch_set_pc(fregs, (unsigned long)func->new_func);
> 
>  unlock:
> -	preempt_enable_notrace();
>  	ftrace_test_recursion_unlock(bit);
>  }

Side note... the comment will eventually conflict with peterz's 
https://lore.kernel.org/all/20210929152429.125997206@infradead.org/.

Miroslav