From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756762Ab3K0Pf1 (ORCPT ); Wed, 27 Nov 2013 10:35:27 -0500 Received: from mail-bk0-f51.google.com ([209.85.214.51]:36247 "EHLO mail-bk0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756675Ab3K0PfX (ORCPT ); Wed, 27 Nov 2013 10:35:23 -0500 Date: Wed, 27 Nov 2013 16:35:19 +0100 From: Ingo Molnar To: Peter Zijlstra Cc: Steven Rostedt , Juri Lelli , tglx@linutronix.de, mingo@redhat.com, oleg@redhat.com, fweisbec@gmail.com, darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch, linux-kernel@vger.kernel.org, claudio@evidence.eu.com, michael@amarulasolutions.com, fchecconi@gmail.com, tommaso.cucinotta@sssup.it, nicola.manica@disi.unitn.it, luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com, paulmck@linux.vnet.ibm.com, raistlin@linux.it, insop.song@gmail.com, liming.wang@windriver.com, jkacur@redhat.com, harald.gustafsson@ericsson.com, vincent.guittot@linaro.org, bruce.ashfield@windriver.com Subject: Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks. Message-ID: <20131127153519.GA26095@gmail.com> References: <1383831828-15501-1-git-send-email-juri.lelli@gmail.com> <1383831828-15501-9-git-send-email-juri.lelli@gmail.com> <20131120163318.10253e43@gandalf.local.home> <5295F711.5010708@gmail.com> <20131127091647.4e16ce53@gandalf.local.home> <20131127142649.GC13532@twins.programming.kicks-ass.net> <20131127143435.GD25043@gmail.com> <20131127145837.GE16796@laptop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131127145837.GE16796@laptop.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Wed, Nov 27, 2013 at 03:34:35PM +0100, Ingo Molnar wrote: > > > > * Peter Zijlstra 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 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