linux-kernel - Re: [PATCH v5 15/37] tracing: Add variable support to hist triggers

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Date:   Fri, 10 Nov 2017 16:28:41 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel.opensrc@...il.com, joelaf@...gle.com,
        mathieu.desnoyers@...icios.com, baohong.liu@...el.com,
        rajvi.jingar@...el.com, julia@...com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v5 15/37] tracing: Add variable support to hist triggers

Hi Tom,

On Thu, Nov 09, 2017 at 02:33:46PM -0600, Tom Zanussi wrote:
> Add support for saving the value of a current event's event field by
> assigning it to a variable that can be read by a subsequent event.
> 
> The basic syntax for saving a variable is to simply prefix a unique
> variable name not corresponding to any keyword along with an '=' sign
> to any event field.
> 
> Both keys and values can be saved and retrieved in this way:
> 
>     # echo 'hist:keys=next_pid:vals=$ts0:ts0=$common_timestamp ...
>     # echo 'hist:timer_pid=common_pid:key=$timer_pid ...'
> 
> If a variable isn't a key variable or prefixed with 'vals=', the
> associated event field will be saved in a variable but won't be summed
> as a value:
> 
>     # echo 'hist:keys=next_pid:ts1=$common_timestamp:...
> 
> Multiple variables can be assigned at the same time:
> 
>     # echo 'hist:keys=pid:vals=$ts0,$b,field2:ts0=$common_timestamp,b=field1 ...
> 
> Multiple (or single) variables can also be assigned at the same time
> using separate assignments:
> 
>     # echo 'hist:keys=pid:vals=$ts0:ts0=$common_timestamp:b=field1:c=field2 ...
> 
> Variables set as above can be used by being referenced from another
> event, as described in a subsequent patch.
> 
> Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
> Signed-off-by: Baohong Liu <baohong.liu@...el.com>
> ---

[SNIP]
> +static int parse_var_defs(struct hist_trigger_data *hist_data)
> +{
> +	char *s, *str, *var_name, *field_str;
> +	unsigned int i, j, n_vars = 0;
> +	int ret = 0;
> +
> +	for (i = 0; i < hist_data->attrs->n_assignments; i++) {
> +		str = hist_data->attrs->assignment_str[i];
> +		for (j = 0; j < TRACING_MAP_VARS_MAX; j++) {
> +			field_str = strsep(&str, ",");
> +			if (!field_str)
> +				break;
> +
> +			var_name = strsep(&field_str, "=");
> +			if (!var_name || !field_str) {
> +				ret = -EINVAL;
> +				goto free;
> +			}
> +
> +			s = kstrdup(var_name, GFP_KERNEL);
> +			if (!s) {
> +				ret = -ENOMEM;
> +				goto free;
> +			}
> +			hist_data->attrs->var_defs.name[n_vars] = s;
> +
> +			s = kstrdup(field_str, GFP_KERNEL);
> +			if (!s) {
> +				kfree(hist_data->attrs->var_defs.name[n_vars]);
> +				ret = -ENOMEM;
> +				goto free;
> +			}
> +			hist_data->attrs->var_defs.expr[n_vars++] = s;
> +
> +			hist_data->attrs->var_defs.n_vars = n_vars;
> +
> +			if (n_vars == TRACING_MAP_VARS_MAX)
> +				goto free;

This will silently discard all variables.  Why not returning an error?
Also I think it should be moved to the beginning of this block..

Thanks,
Namhyung


> +		}
> +	}
> +
> +	return ret;
> + free:
> +	free_var_defs(hist_data);
> +
> +	return ret;
> +}