Re: [PATCH v3 15/28] module: Use RCU in all users of __module_text_address().

[Benjamin Berg <benjamin@sipsolutions.net>]

Hi,

On Wed, 2025-01-08 at 10:04 +0100, Sebastian Andrzej Siewior wrote:
> __module_text_address() can be invoked within a RCU section, there is no
> requirement to have preemption disabled.
> 
> Replace the preempt_disable() section around __module_text_address()
> with RCU.

Unfortunately, this patch causes a performance regression for us. The
trouble is that we enable kmemleak and run trace-cmd so a lot of stack
traces need to be collected. Obviously, we also have lockdep enabled.

Now, combine this with the UML stack dumping code calling into
__kernel_text_address a lot[1] and it really has a relevant performance
impact. I saw the kernel spending 40% of its own CPU time just on the
lock in is_module_text_address.

Maybe kernel_text_address should leave the RCU handling to the caller
and assume that the RCU read lock is already taken?

Benjamin

[1] The UM arch dump_stack function reads every "unsigned long" on the
stack and tests it using __kernel_text_address.

> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  kernel/module/main.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 80877741ac7e5..6a99076146cbc 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -823,13 +823,12 @@ void symbol_put_addr(void *addr)
>  
>  	/*
>  	 * Even though we hold a reference on the module; we still
> need to
> -	 * disable preemption in order to safely traverse the data
> structure.
> +	 * RCU read section in order to safely traverse the data
> structure.
>  	 */
> -	preempt_disable();
> +	guard(rcu)();
>  	modaddr = __module_text_address(a);
>  	BUG_ON(!modaddr);
>  	module_put(modaddr);
> -	preempt_enable();
>  }
>  EXPORT_SYMBOL_GPL(symbol_put_addr);
>  
> @@ -3694,20 +3693,15 @@ struct module *__module_address(unsigned long
> addr)
>   */
>  bool is_module_text_address(unsigned long addr)
>  {
> -	bool ret;
> -
> -	preempt_disable();
> -	ret = __module_text_address(addr) != NULL;
> -	preempt_enable();
> -
> -	return ret;
> +	guard(rcu)();
> +	return __module_text_address(addr) != NULL;
>  }
>  
>  /**
>   * __module_text_address() - get the module whose code contains an
> address.
>   * @addr: the address.
>   *
> - * Must be called with preempt disabled or module mutex held so that
> + * Must be called within RCU read section or module mutex held so
> that
>   * module doesn't get freed during this.
>   */
>  struct module *__module_text_address(unsigned long addr)