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

[Jiri Olsa]

On Mon, Oct 18, 2021 at 10:10:15PM -0400, Steven Rostedt wrote:
> On Sat, 16 Oct 2021 13:39:55 +0200
> Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> 
> > On Fri, Oct 15, 2021 at 10:05:09AM -0400, Steven Rostedt wrote:
> > > On Fri, 15 Oct 2021 14:05:25 +0200
> > > Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > >   
> > > > 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 ;-)  
> > > 
> > > If you want, I'll apply this as a separate change on top of your patch set.
> > > As I don't see anything wrong with your current code.
> > > 
> > > And when you are satisfied with this, just give me a "tested-by" and I'll
> > > push it too.  
> > 
> > sounds great, thanks
> > jirka
> 
> Would you want to ack/review this?

hum, do you have it in some branch already? I'm getting:

patching file kernel/trace/ftrace.c
Hunk #1 succeeded at 5521 with fuzz 1 (offset -40 lines).
Hunk #2 FAILED at 5576.
Hunk #3 succeeded at 5557 (offset -44 lines).
1 out of 3 hunks FAILED -- saving rejects to file kernel/trace/ftrace.c.rej


when trying to apply on top of my changes

thanks,
jirka

> 
> -- Steve
> 
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> Subject: [PATCH] ftrace/direct: Do not disable when switching direct callers
> 
> Currently to switch a set of "multi" direct trampolines from one
> trampoline to another, a full shutdown of the current set needs to be
> done, followed by an update to what trampoline the direct callers would
> call, and then re-enabling the callers. This leaves a time when the
> functions will not be calling anything, and events may be missed.
> 
> Instead, use a trick to allow all the functions with direct trampolines
> attached will always call either the new or old trampoline while the
> switch is happening. To do this, first attach a "dummy" callback via
> ftrace to all the functions that the current direct trampoline is attached
> to. This will cause the functions to call the "list func" instead of the
> direct trampoline. The list function will call the direct trampoline
> "helper" that will set the function it should call as it returns back to
> the ftrace trampoline.
> 
> At this moment, the direct caller descriptor can safely update the direct
> call trampoline. The list function will pick either the new or old
> function (depending on the memory coherency model of the architecture).
> 
> Now removing the dummy function from each of the locations of the direct
> trampoline caller, will put back the direct call, but now to the new
> trampoline.
> 
> A better visual is:
> 
> [ Changing direct call from my_direct_1 to my_direct_2 ]
> 
>   <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
> 
>  ||||||||||||||||||||
>  vvvvvvvvvvvvvvvvvvvv
> 
>   <traced_func>:
>      call my_direct_2
> 
> Link: https://lore.kernel.org/all/20211014162819.5c85618b@xxxxxxxxxxxxxxxxxx/
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  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
>