linux-kernel - Re: [PATCH v2 10/11] perf expr: print a debug message for division by zero

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Date:   Thu, 23 Apr 2020 07:16:48 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Haiyan Song <haiyanx.song@...el.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        John Garry <john.garry@...wei.com>,
        Leo Yan <leo.yan@...aro.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Paul Clarke <pc@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 10/11] perf expr: print a debug message for division by zero

On Thu, Apr 23, 2020 at 4:28 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Wed, Apr 22, 2020 at 03:04:29PM -0700, Ian Rogers wrote:
> > If an expression yields 0 and is then divided-by/modulus-by then the
> > parsing aborts. Add a debug error message to better enable debugging
> > when this happens.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/perf/util/expr.y | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/expr.y b/tools/perf/util/expr.y
> > index 54260094b947..21e82a1e11a2 100644
> > --- a/tools/perf/util/expr.y
> > +++ b/tools/perf/util/expr.y
> > @@ -103,8 +103,18 @@ expr:      NUMBER
> >       | expr '+' expr         { $$ = $1 + $3; }
> >       | expr '-' expr         { $$ = $1 - $3; }
> >       | expr '*' expr         { $$ = $1 * $3; }
> > -     | expr '/' expr         { if ($3 == 0) YYABORT; $$ = $1 / $3; }
> > -     | expr '%' expr         { if ((long)$3 == 0) YYABORT; $$ = (long)$1 % (long)$3; }
> > +     | expr '/' expr         { if ($3 == 0) {
> > +                                     pr_debug("division by zero\n");
> > +                                     YYABORT;
> > +                               }
> > +                               $$ = $1 / $3;
> > +                             }
> > +     | expr '%' expr         { if ((long)$3 == 0) {
>
> is that (long) cast necessary? it's missing for the '/' case

Andi Kleen could say for sure :-) From my observation, the values are
typically doubles and definitely need to be in the divide case. There
is also code handling them as longs such as |, &, ^:
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/tools/perf/util/expr.y#n100
There's an experiment to see if all of the long handling code could be
removed. This change isn't modifying the existing behavior.

Thanks,
Ian

> jirka
>
> > +                                     pr_debug("division by zero\n");
> > +                                     YYABORT;
> > +                               }
> > +                               $$ = (long)$1 % (long)$3;
> > +                             }
> >       | '-' expr %prec NEG    { $$ = -$2; }
> >       | '(' if_expr ')'       { $$ = $2; }
> >       | MIN '(' expr ',' expr ')' { $$ = $3 < $5 ? $3 : $5; }
> > --
> > 2.26.2.303.gf8c07b1a785-goog
> >
>