linux-kernel - Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks.

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Message-ID: <20131127153519.GA26095@gmail.com>
Date:	Wed, 27 Nov 2013 16:35:19 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	harald.gustafsson@...csson.com, vincent.guittot@...aro.org,
	bruce.ashfield@...driver.com
Subject: Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks.


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Nov 27, 2013 at 03:34:35PM +0100, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Wed, Nov 27, 2013 at 09:16:47AM -0500, Steven Rostedt wrote:
> > > > On Wed, 27 Nov 2013 14:43:45 +0100
> > > > Juri Lelli <juri.lelli@...il.com> wrote:
> > > > diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
> > > > > index f76f8d6..ad94604 100644
> > > > > --- a/kernel/trace/trace_selftest.c
> > > > > +++ b/kernel/trace/trace_selftest.c
> > > > > @@ -1023,16 +1023,16 @@ trace_selftest_startup_nop(struct tracer *trace, struct trace_array *tr)
> > > > >  static int trace_wakeup_test_thread(void *data)
> > > > >  {
> > > > >  	/* Make this a -deadline thread */
> > > > > -	struct sched_param2 paramx = {
> > > > > +	static const struct sched_param2 param = {
> > > > >  		.sched_priority = 0,
> > > > > +		.sched_flags = 0,
> > > > >  		.sched_runtime = 100000ULL,
> > > > >  		.sched_deadline = 10000000ULL,
> > > > >  		.sched_period = 10000000ULL
> > > > > -		.sched_flags = 0
> > > > 
> > > > Assigning structures like this, you don't need to set the zero fields.
> > > > all fields not explicitly stated, are set to zero.
> > > 
> > > Only because its static. Otherwise unnamed members have indeterminate
> > > value after initialization.
> > 
> > I think for 'struct' C will initialize them to zero, even if they are 
> > not mentioned and even if they are on the stack.
> > 
> > It will only be indeterminate when it's not initialized at all.
> 
> Language spec: ISO/IEC 9899:1999 (aka C99) section 6.7.8 point 9
> says:
> 
> 9. Except where explicitly stated otherwise, for the purpose of this
> subclause unnamed members of objects of structure and union type do no
> participate in initialization. Unnamed members of structure objects have
> indeterminate value even after initialization.
> 
> Later points (notably 21) make such an exception for aggregate objects
> of static storage.
> 
> Of course, its entirely possible I read the thing wrong; its 31 points
> detailing the initialization of objects.

So why does GCC then behave like this:

triton:~> cat test.c

struct foo {
        int a;
        int b;
};

int main(void)
{
        struct foo x = { .a = 1 };

        return x.b;
}

triton:~> gcc -Wall -Wextra -O2 -o test test.c; ./test; echo $?
0

I'd expect -Wall -Wextra to warn about as trivial as the uninitialized 
variable use that you argue happens.

I'd also expect it to not return 0 but some random value on the stack 
(which is most likely not 0).

Thanks,

	Ingo
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