linux-kernel - Re: [PATCH] tracing: add trace_seq

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Date: Mon, 22 Jan 2024 17:10:24 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Ricardo B. Marliere" <ricardo@...liere.net>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: add trace_seq_reset function

On Mon, 22 Jan 2024 15:22:25 -0300
"Ricardo B. Marliere" <ricardo@...liere.net> wrote:

> Currently, trace_seq_init may be called many times with the intent of
> resetting the buffer. Add a function trace_seq_reset that does that and
> replace the relevant occurrences to use it instead.
> 

Hi Ricardo!

It's also OK to add to the commit log that the goal of this is to later
extend trace_seq to be more flexible in the size of the buffer it holds. To
do that, we need to differentiate between initializing a trace_seq and just
resetting it.


> Signed-off-by: Ricardo B. Marliere <ricardo@...liere.net>
> ---
>  include/linux/trace_seq.h    | 8 ++++++++
>  include/trace/trace_events.h | 2 +-
>  kernel/trace/trace.c         | 8 ++++----
>  kernel/trace/trace_seq.c     | 2 +-
>  4 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/trace_seq.h b/include/linux/trace_seq.h
> index 9ec229dfddaa..c28e0ccb50bd 100644
> --- a/include/linux/trace_seq.h
> +++ b/include/linux/trace_seq.h
> @@ -29,6 +29,14 @@ trace_seq_init(struct trace_seq *s)
>  	s->readpos = 0;
>  }
>  
> +static inline void
> +trace_seq_reset(struct trace_seq *s)
> +{
> +	seq_buf_clear(&s->seq);
> +	s->full = 0;
> +	s->readpos = 0;
> +}
> +
>  /**
>   * trace_seq_used - amount of actual data written to buffer
>   * @s: trace sequence descriptor
> diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
> index c2f9cabf154d..2bc79998e5ab 100644
> --- a/include/trace/trace_events.h
> +++ b/include/trace/trace_events.h
> @@ -227,7 +227,7 @@ trace_raw_output_##call(struct trace_iterator *iter, int flags,		\
>  									\
>  	field = (typeof(field))entry;					\
>  									\
> -	trace_seq_init(p);						\
> +	trace_seq_reset(p);						\
>  	return trace_output_call(iter, #call, print);			\

Note, p = &iter->tmp_seq

where it has never been initialized. To do this, we need to add:

	trace_seq_init(&iter->tmp_seq);

where ever iter is created. You need to look at all the locations where
iter is created ("iter =") and differentiate where it is first used from
just passing pointers around.

The iter = kzalloc() will be easy, but for example, both seq and tmp_seq
need to be initialized in tracing_buffers_open().

Perhaps we need a:

	if (WARN_ON_ONCE(!s->seq.size))
		seq_buf_init(&s->seq, s->buffer, TRACE_SEQ_BUFFER_SIZE);
	else
		seq_buf_clear(&s->seq);


in the trace_seq_reset() to catch any place that doesn't have it
initialized yet.

-- Steve

>  }									\
>  static struct trace_event_functions trace_event_type_funcs_##call = {	\
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 46dbe22121e9..9147f3717b9a 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6946,7 +6946,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>  	/* reset all but tr, trace, and overruns */
>  	trace_iterator_reset(iter);
>  	cpumask_clear(iter->started);
> -	trace_seq_init(&iter->seq);
> +	trace_seq_reset(&iter->seq);
>  
>  	trace_event_read_lock();
>  	trace_access_lock(iter->cpu_file);
> @@ -6993,7 +6993,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>  	/* Now copy what we have to the user */
>  	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
>  	if (iter->seq.readpos >= trace_seq_used(&iter->seq))
> -		trace_seq_init(&iter->seq);
> +		trace_seq_reset(&iter->seq);
>  
>  	/*
>  	 * If there was nothing to send to user, in spite of consuming trace
> @@ -7125,7 +7125,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
>  		spd.partial[i].offset = 0;
>  		spd.partial[i].len = trace_seq_used(&iter->seq);
>  
> -		trace_seq_init(&iter->seq);
> +		trace_seq_reset(&iter->seq);
>  	}
>  
>  	trace_access_unlock(iter->cpu_file);
> @@ -10274,7 +10274,7 @@ trace_printk_seq(struct trace_seq *s)
>  
>  	printk(KERN_TRACE "%s", s->buffer);
>  
> -	trace_seq_init(s);
> +	trace_seq_reset(s);
>  }
>  
>  void trace_init_global_iter(struct trace_iterator *iter)
> diff --git a/kernel/trace/trace_seq.c b/kernel/trace/trace_seq.c
> index c158d65a8a88..741b2f3d76c0 100644
> --- a/kernel/trace/trace_seq.c
> +++ b/kernel/trace/trace_seq.c
> @@ -59,7 +59,7 @@ int trace_print_seq(struct seq_file *m, struct trace_seq *s)
>  	 * do something else with the contents.
>  	 */
>  	if (!ret)
> -		trace_seq_init(s);
> +		trace_seq_reset(s);
>  
>  	return ret;
>  }