Subject: Re: [PATCH] debug rcu head support !PREEMPT config
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org,
        niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
        Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com,
        darren@dvhart.com
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Date: Wed, 23 Feb 2011 10:27:07 -0500
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On Wed, 2011-02-23 at 10:13 -0500, Mathieu Desnoyers wrote:
> Remove DEBUG_RCU_HEAD dependency on PREEMPT config. Handle the unability to
> detect if within a RCU read-side critical section by never performing any
> attempt to recover from a failure situation in the fixup handlers. Just print
> the warnings.
> 
> This patch is only compile-tested.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
>  kernel/rcupdate.c |   17 +++++++++++++++++
>  lib/Kconfig.debug |    2 +-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6-lttng/lib/Kconfig.debug
> ===================================================================
> --- linux-2.6-lttng.orig/lib/Kconfig.debug
> +++ linux-2.6-lttng/lib/Kconfig.debug
> @@ -313,7 +313,7 @@ config DEBUG_OBJECTS_WORK
>  
>  config DEBUG_OBJECTS_RCU_HEAD
>  	bool "Debug RCU callbacks objects"
> -	depends on DEBUG_OBJECTS && PREEMPT
> +	depends on DEBUG_OBJECTS
>  	help
>  	  Enable this to turn on debugging of RCU list heads (call_rcu() usage).
>  
> Index: linux-2.6-lttng/kernel/rcupdate.c
> ===================================================================
> --- linux-2.6-lttng.orig/kernel/rcupdate.c
> +++ linux-2.6-lttng/kernel/rcupdate.c
> @@ -142,7 +142,14 @@ static int rcuhead_fixup_init(void *addr
>  		 * Ensure that queued callbacks are all executed.
>  		 * If we detect that we are nested in a RCU read-side critical
>  		 * section, we should simply fail, otherwise we would deadlock.
> +		 * In !PREEMPT configurations, there is no way to tell if we are
> +		 * in a RCU read-side critical section or not, so we never
> +		 * attempt any fixup and just print a warning.
>  		 */
> +#ifndef CONFIG_PREEMPT
> +		WARN_ON(1);
> +		return 0;
> +#endif
>  		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
>  		    irqs_disabled()) {
>  			WARN_ON(1);
> @@ -184,7 +191,14 @@ static int rcuhead_fixup_activate(void *
>  		 * Ensure that queued callbacks are all executed.
>  		 * If we detect that we are nested in a RCU read-side critical
>  		 * section, we should simply fail, otherwise we would deadlock.
> +		 * In !PREEMPT configurations, there is no way to tell if we are
> +		 * in a RCU read-side critical section or not, so we never
> +		 * attempt any fixup and just print a warning.
>  		 */
> +#ifndef CONFIG_PREEMPT
> +		WARN_ON(1);
> +		return 0;
> +#endif
>  		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
>  		    irqs_disabled()) {
>  			WARN_ON(1);
> @@ -214,6 +228,9 @@ static int rcuhead_fixup_free(void *addr
>  		 * Ensure that queued callbacks are all executed.
>  		 * If we detect that we are nested in a RCU read-side critical
>  		 * section, we should simply fail, otherwise we would deadlock.
> +		 * In !PREEMPT configurations, there is no way to tell if we are
> +		 * in a RCU read-side critical section or not, so we never
> +		 * attempt any fixup and just print a warning.
>  		 */
>  #ifndef CONFIG_PREEMPT
>  		WARN_ON(1);

Hmm, I wonder if s/WARN_ON/WARN_ON_ONCE/g is in order. Why spam the
console if it happens to trigger all the time?

-- Steve


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