linux-kernel - Re: [PATCH] rcutree: Fix v3.4-rc2-rt2 build break

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Date:	Fri, 13 Apr 2012 01:13:38 +0200
From:	John Kacur <jkacur@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH] rcutree: Fix v3.4-rc2-rt2 build break

On Thu, Apr 12, 2012 at 9:46 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, 11 Apr 2012, John Kacur wrote:
>
> > Fix build break of the following types.
> >
> > linux-rt/kernel/rcutree_plugin.h: In function
> > ‘print_cpu_stall_fast_no_hz’:
> > linux-rt/kernel/rcutree_plugin.h:2195: error: ‘rcu_idle_gp_timer’
> > undeclared (first use in this function)
> > linux-rt/kernel/rcutree_plugin.h:2195: error: (Each undeclared
> > identifier is reported only once
> > linux-rt/kernel/rcutree_plugin.h:2195: error: for each function it
> > appears in.)
> >
> > The build break only occurs with the PREEMPT_RT_FULL patch applied,
> > however
> > the patch is meant to go upstream and be applied to v3.4-rc2 as well
> > because
> > it makes the code more legible there, and will reduce the number of
> > places
> > where #ifdef PREEMPT_RT_FULL is required should that go upstream
> > someday.
>
> -ENOPARSE
>
> I really have no idea what the patch is solving and which particular
> combination of config items is causing the above.

The config combination that causes a build break is

CONFIG_RCU_FAST_NO_HZ
CONFIG_PREEMPT_RT_FULL
CONFIG_RCU_CPU_STALL_INFO

The patch does the following

#if !defined(CONFIG_RCU_FAST_NO_HZ) || defined(CONFIG_PREEMPT_RT_FULL
        The patch moves the skeleton versions of the functions to this section
#else
        Versions of the functions that were causing a build break.
#endif

>
> If you think that the patch should go upstream then please send a
> separate one with a changelog which explains the simplifcation.

Applying the patch upstream causes no functional changes there.
I can try resubmitting a patch for upstream with a clearer changelog.
Or I perhaps I should dig deeper and try to simplify more for
upstream, but this is the simplest way I saw to fix the build break
for rt

>
> I agree that this ifdef maze can do with simplification, but providing
> a changelog which tells nothing at all does not make it easier to grok
> the problem and understand what's simplified.
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