linux-kernel - Re: [PATCH 7/8] ftrace: Add multi direct modify interface

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Message-ID: <YWluhdDMfkNGwlhz@krava>
Date:   Fri, 15 Oct 2021 14:05:25 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH 7/8] ftrace: Add multi direct modify interface

On Thu, Oct 14, 2021 at 04:28:19PM -0400, Steven Rostedt wrote:
> On Fri,  8 Oct 2021 11:13:35 +0200
> Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > +	/*
> > +	 * Shutdown the ops, change 'direct' pointer for each
> > +	 * ops entry in direct_functions hash and startup the
> > +	 * ops back again.
> > +	 *
> > +	 * Note there is no callback called for @ops object after
> > +	 * this ftrace_shutdown call until ftrace_startup is called
> > +	 * later on.
> > +	 */
> > +	err = ftrace_shutdown(ops, 0);
> > +	if (err)
> > +		goto out_unlock;
> 
> I believe I said before that we can do this by adding a stub ops that match
> all the functions with the direct ops being modified. This will cause the
> loop function to be called, which will call the direct function helper,
> which will then call the direct function that is found. That is, there is
> no "pause" in calling the direct callers. Either the old direct is called,
> or the new one. When the function returns, all are calling the new one.
> 
> That is, instead of:
> 
> [ Changing direct call from my_direct_1 to my_direct_2 ]
> 
>   <traced_func>:
>      call my_direct_1
> 
>  ||||||||||||||||||||
>  vvvvvvvvvvvvvvvvvvvv
> 
>   <traced_func>:
>      nop
> 
>  ||||||||||||||||||||
>  vvvvvvvvvvvvvvvvvvvv
> 
>   <traced_func>:
>      call my_direct_2
> 
> 
> We have it do:
> 
>   <traced_func>:
>      call my_direct_1
> 
>  ||||||||||||||||||||
>  vvvvvvvvvvvvvvvvvvvv
> 
>   <traced_func>:
>      call ftrace_caller
> 
> 
>   <ftrace_caller>:
>     [..]
>     call ftrace_ops_list_func
> 
> 
> ftrace_ops_list_func()
> {
> 	ops->func() -> direct_helper -> set rax to my_direct_1 or my_direct_2
> }
> 
>    call rax (to either my_direct_1 or my_direct_2

nice! :) I did not see that as a problem and something that can be
done later, thanks for doing this

> 
>  ||||||||||||||||||||
>  vvvvvvvvvvvvvvvvvvvv
> 
>   <traced_func>:
>      call my_direct_2
> 
> 
> I did this on top of this patch:

ATM I'm bit stuck on the bpf side of this whole change, I'll test
it with my other changes when I unstuck myself ;-)

thanks,
jirka

> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  kernel/trace/ftrace.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 30120342176e..7ad1e8ae5855 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5561,8 +5561,12 @@ EXPORT_SYMBOL_GPL(unregister_ftrace_direct_multi);
>   */
>  int modify_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
>  {
> -	struct ftrace_hash *hash = ops->func_hash->filter_hash;
> +	struct ftrace_hash *hash;
>  	struct ftrace_func_entry *entry, *iter;
> +	static struct ftrace_ops tmp_ops = {
> +		.func		= ftrace_stub,
> +		.flags		= FTRACE_OPS_FL_STUB,
> +	};
>  	int i, size;
>  	int err;
>  
> @@ -5572,21 +5576,22 @@ int modify_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
>  		return -EINVAL;
>  
>  	mutex_lock(&direct_mutex);
> -	mutex_lock(&ftrace_lock);
> +
> +	/* Enable the tmp_ops to have the same functions as the direct ops */
> +	ftrace_ops_init(&tmp_ops);
> +	tmp_ops.func_hash = ops->func_hash;
> +
> +	err = register_ftrace_function(&tmp_ops);
> +	if (err)
> +		goto out_direct;
>  
>  	/*
> -	 * Shutdown the ops, change 'direct' pointer for each
> -	 * ops entry in direct_functions hash and startup the
> -	 * ops back again.
> -	 *
> -	 * Note there is no callback called for @ops object after
> -	 * this ftrace_shutdown call until ftrace_startup is called
> -	 * later on.
> +	 * Now the ftrace_ops_list_func() is called to do the direct callers.
> +	 * We can safely change the direct functions attached to each entry.
>  	 */
> -	err = ftrace_shutdown(ops, 0);
> -	if (err)
> -		goto out_unlock;
> +	mutex_lock(&ftrace_lock);
>  
> +	hash = ops->func_hash->filter_hash;
>  	size = 1 << hash->size_bits;
>  	for (i = 0; i < size; i++) {
>  		hlist_for_each_entry(iter, &hash->buckets[i], hlist) {
> @@ -5597,10 +5602,12 @@ int modify_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
>  		}
>  	}
>  
> -	err = ftrace_startup(ops, 0);
> +	/* Removing the tmp_ops will add the updated direct callers to the functions */
> +	unregister_ftrace_function(&tmp_ops);
>  
>   out_unlock:
>  	mutex_unlock(&ftrace_lock);
> + out_direct:
>  	mutex_unlock(&direct_mutex);
>  	return err;
>  }
> -- 
> 2.31.1
>