linux-kernel - Re: [PATCH 4/5] perf tools: Propagate error info from tp

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Date:	Thu, 10 Sep 2015 11:16:28 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Raphaël Beamonte <raphael.beamonte@...il.com>
Subject: Re: [PATCH 4/5] perf tools: Propagate error info from tp_format

Em Thu, Sep 10, 2015 at 10:24:52AM +0200, Jiri Olsa escreveu:
> On Wed, Sep 09, 2015 at 05:58:13PM -0300, Arnaldo Carvalho de Melo wrote:
> 
> SNIP
> 
> > This kind of stuff is ok, as evsel is a local variable and you kept the
> > interface for perf_evsel__syscall_newtp(), i.e. it returns NULL if a new
> > evsel can't be instantiated.
> > 
> > Ok, but that is a different interface than the one used by
> > perf_evsel__newtp(), that also instantiates a new evsel.
> > 
> > So when one thinks about "foo__new()" we now need to check which one of
> > the two interfaces it uses, if err.h or if the old NULL based failure
> > reporting one.
> > 
> > Double tricky if it is foo__new() and foo__new_variant(), as
> > perf_evsel__syscall_newtp() and perf_evsel__newtp(), i.e. both will
> > return a "struct perf_evsel" instance, but one using err.h, the other
> > use NULL.
> > 
> > Ok, you marked the ones using a comment, wonder if we couldn't use
> > 'sparse' somehow here, is it used to check IS_ERR() usage in the kernel?
> 
> hum, not sure.. will check ;-)
> 
> at least we could mark related functions with __must_check
> to force the return value check

Right, that helps a bit, but not when the test _is already there_,
against NULL.

That is why I thought about sparse, if it was used in the kernel somehow
to check for this, guess either it would notice ERR_PTR using routines
and then auto-mark them for checking if they are being tested using
IS_ERR() or plain NULL, will check, later...

- Arnaldo
 
> > 
> > Ah, but what about this in trace__event_handler() in builtin-trace.c?
> >  
> >         if (evsel->tp_format) {
> >                 event_format__fprintf(evsel->tp_format, sample->cpu,
> >                                       sample->raw_data, sample->raw_size,
> >                                       trace->output);
> >         }
> > 
> > 
> > Don't we have to use IS_ERR() here? Ok, no, because if setting up
> > evsel->tp_format fails, then that evsel will be destroyed and
> > perf_evsel__newtp() will return ERR_PTR(), so it is ok not no use
> > ERR_PTR(evsel->tp_format) because it will only be != NULL when it was
> > successfully set up.
> > 
> > But then, in perf_evsel__newtp_idx if zalloc() fails we will not return
> > ERR_PTR(), but instead NULL, a-ha, this one seems to be a real bug, no?
> 
> hate those allocations in declarations.. never do any good ;-)
> 
> yep, NULL is not an error, so it's real bug, attached patch should fix it
> 
> thanks,
> jirka
> 
> 
> ---
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 08c20ee4e27d..162973bec713 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -256,7 +256,7 @@ struct perf_evsel *perf_evsel__newtp_idx(const char *sys, const char *name, int
>  		perf_evsel__init(evsel, &attr, idx);
>  	}
>  
> -	return evsel;
> +	return evsel ?: ERR_PTR(err);
>  
>  out_free:
>  	zfree(&evsel->name);
--
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