Re: [Qemu-devel] [PATCH v4 4/7] trace: [monitor] Use new event control interface

[Lluís Vilanova]

Lluís Vilanova writes:

> Stefan Hajnoczi writes:
>> On Tue, May 8, 2012 at 3:38 PM, Lluís Vilanova <address@hidden> wrote:
>>> Signed-off-by: Lluís Vilanova <address@hidden>
>>> ---
>>>  monitor.c |   15 ++++++++++++---
>>>  1 file changed, 12 insertions(+), 3 deletions(-)
>>> 
>>> diff --git a/monitor.c b/monitor.c
>>> index 8946a10..86c2538 100644
>>> --- a/monitor.c
>>> +++ b/monitor.c
>>> @@ -625,10 +625,19 @@ static void do_trace_event_set_state(Monitor *mon, 
>>> const QDict *qdict)
>>>  {
>>>     const char *tp_name = qdict_get_str(qdict, "name");
>>>     bool new_state = qdict_get_bool(qdict, "option");
>>> -    int ret = trace_event_set_state(tp_name, new_state);
>>> 
>>> -    if (!ret) {
>>> -        monitor_printf(mon, "unknown event name \"%s\"\n", tp_name);
>>> +    if (trace_event_is_pattern(tp_name)) {
>>> +        TraceEvent *ev = NULL;
>>> +        while ((ev = trace_event_pattern(tp_name, ev)) != NULL) {
>>> +            trace_event_set_state_dynamic(ev, new_state);
>>> +        }
>>> +    } else {
>>> +        TraceEvent *ev = trace_event_name(tp_name);
>>> +        if (ev == NULL) {
>>> +            monitor_printf(mon, "unknown event name \"%s\"\n", tp_name);
>>> +        } else {
>>> +            trace_event_set_state_dynamic(ev, new_state);
>>> +        }

>> Why check for a pattern and split the code in two?  How about just:

>> while ((ev = trace_event_pattern(tp_name, ev)) != NULL) {
>> ...
>> }

>> That should cover both the single trace event name case and the wildcard 
>> case.

> That's true... it's just that somehow I thought it was abusive to use pattern
> matching on a string without patterns :)

When going through the code to add your comments I realized why it made sense to
use 'trace_event_name' instead of 'trace_event_pattern'.

In the case the string contains no pattern, not finding a result is an error in
'trace_backend_init_events' (when reading the list of events given in the
commandline file), as well as in 'do_trace_event_set_state' (when setting the
state from the monitor, although here the error is not fatal).

In comparison, setting by pattern never fails.


Lluis

-- 
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth