linux-kernel - Re: [RFC PATCH 1/2] rcu: sysctl: Panic on RCU Stall

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Date:	Tue, 31 May 2016 19:34:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Daniel Bristot de Oliveira <daolivei@...hat.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [RFC PATCH 1/2] rcu: sysctl: Panic on RCU Stall

On Tue, May 31, 2016 at 07:49:41PM -0300, Daniel Bristot de Oliveira wrote:
> On 05/31/2016 04:23 PM, Josh Triplett wrote:
> 
> Hi Josh,
> 
> > Sorry, realized something else a moment after sending: I don't think
> > this will build if you use the tiny RCU implementation.  That
> > implementation *does* support tracing, and if you enable tracing,
> > you'll have CONFIG_RCU_STALL_COMMON=y, but you won't build tree.c where
> > the variable definition lives.  So, the sysctl code will reference a
> > variable that doesn't exist.
> 
> Good catch!
> 
> [root@f23 linux]# egrep '(TINY_RCU|RCU_TRACE)' .config
> CONFIG_TINY_RCU=y
> # CONFIG_TREE_RCU_TRACE is not set
> CONFIG_RCU_TRACE=y
> [root@f23 linux]# make
> ...
> kernel/built-in.o:(.data+0x4688): undefined reference to `sysctl_panic_on_rcu_stall'
> Makefile:937: recipe for target 'vmlinux' failed
> make: *** [vmlinux] Error 1
> 
> 
> How about this change in the condition:
> -#ifdef CONFIG_RCU_STALL_COMMON
> +#if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)

Looks plausible to me.  But what I would trust quite a bit more than my
own opinion would be successful builds for all three configurations.  ;-)

							Thanx, Paul

> It worked fine here, but I may be missing something, so I better
> check with the experts :-).
> 
> Thanks for reviewing the patch set!
> 
> -- Daniel
>