linux-kernel - Re: [RFC PATCH v2 06/20] tracing/filters: Optimise scalar vs cpumask filtering when the user mask is a single CPU

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Message-ID: <xhsmhy1iwq9ou.mognet@vschneid.remote.csb>
Date:   Mon, 31 Jul 2023 18:20:49 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Dan Carpenter <dan.carpenter@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
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Subject: Re: [RFC PATCH v2 06/20] tracing/filters: Optimise scalar vs
 cpumask filtering when the user mask is a single CPU

On 31/07/23 19:03, Dan Carpenter wrote:
> On Mon, Jul 31, 2023 at 11:54:53AM -0400, Steven Rostedt wrote:
>> On Mon, 31 Jul 2023 15:07:52 +0300
>> Dan Carpenter <dan.carpenter@...aro.org> wrote:
>>
>> > On Sat, Jul 29, 2023 at 03:55:47PM -0400, Steven Rostedt wrote:
>> > > > @@ -1761,6 +1761,11 @@ static int parse_pred(const char *str, void *data,
>> > > >                                FILTER_PRED_FN_CPUMASK;
>> > > >                } else if (field->filter_type == FILTER_CPU) {
>> > > >                        pred->fn_num = FILTER_PRED_FN_CPU_CPUMASK;
>> > > > +		} else if (single) {
>> > > > +			pred->op = pred->op == OP_BAND ? OP_EQ : pred->op;
>> > >
>> > > Nit, the above can be written as:
>> > >
>> > >                  pred->op = pret->op != OP_BAND ? : OP_EQ;
>> > >
>> >
>> > Heh.  Those are not equivalent.  The right way to write this is:
>>
>> You mean because of my typo?
>
> No, I hadn't seen the s/pred/pret/ typo.  Your code does:
>
>       if (pred->op != OP_BAND)
>               pred->op = true;
>       else
>               pred->op OP_EQ;
>
> Realy we should probably trigger a static checker warning any time
> someone does a compare operations as part of a "x = comparison ?: bar;
> Years ago, someone asked me to do that with regards to error codes like:
>
>       return ret < 0 ?: -EINVAL;
>
> but I don't remember the results.
>

FWIW this is caught by GCC:

     error: the omitted middle operand in ?: will always be ‘true’, suggest explicit middle operand [-Werror=parentheses]
     pred->op = pred->op != OP_BAND ? : OP_EQ;


> regards,
> dan carpenter