linux-kernel - Re: [PATCH v2 3/5] tracing: Fix operator precedence for hist triggers expression

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Date:   Wed, 20 Oct 2021 11:48:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     surenb@...gle.com, hridya@...gle.com, namhyung@...nel.org,
        kernel-team@...roid.com, Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 3/5] tracing: Fix operator precedence for hist
 triggers expression

On Tue, 19 Oct 2021 18:31:40 -0700
Kalesh Singh <kaleshsingh@...gle.com> wrote:

> @@ -2391,60 +2460,61 @@ static int check_expr_operands(struct trace_array *tr,
>  static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
>  				     struct trace_event_file *file,
>  				     char *str, unsigned long flags,
> -				     char *var_name, unsigned int level)
> +				     char *var_name, unsigned int *n_subexprs)
>  {
>  	struct hist_field *operand1 = NULL, *operand2 = NULL, *expr = NULL;
>  	unsigned long operand_flags;
>  	int field_op, ret = -EINVAL;
>  	char *sep, *operand1_str;
>  
> -	if (level > 3) {
> +	if (*n_subexprs > 3) {

Why limit the sub expressions, and not just keep the limit of the level of
recursion. We allow 3 levels of recursion, but we could have more than 3
sub expressions.


If we have:  a * b + c / d - e * f / h

It would break down into:
              -
       +            /
   *       /     *     h
 a   b   c  d  e  f


Which I believe is 6 "sub expressions", but never goes more than three deep
in recursion:

   "a * b + c / d - e * f / h"

Step 1:

  op = "-"
  operand1 = "a * b + c / d"
  operand2 = "e * f / h"

Process operand1: (recursion level 1)

  op = "+"
  operand1a = "a * b"
  operand2a = "c / d"

Process operand1a: (recursion level 2)

  op = "*"
  operand1b = "a"
  operand2b = "b"

return;

Process operand1b: (recursion level 2)

  op = "/"
  operand1b = "c"
  operand2b = "d"

return;

return;

Process operand2: (recursion level 1)

  op = "/"
  operand1c = "e * f"
  operand2c = "h"

Process operand1c: (recursion level 2)

  op = "*"
  operand1c = "e"
  operand2c = "f"

return;

return;



> +
> +	/* LHS of string is an expression e.g. a+b in a+b+c */
> +	operand1 = parse_expr(hist_data, file, operand1_str, operand_flags, NULL, n_subexprs);
>  	if (IS_ERR(operand1)) {
>  		ret = PTR_ERR(operand1);
>  		operand1 = NULL;

I wonder if we should look for optimizations, in case of operand1 and
operand2 are both constants?

Just perform the function, and convert it into a constant as well.

-- Steve