linux-kernel - Re: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU

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Date:	Tue, 22 Feb 2011 22:23:29 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com
Subject: Re: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why
 DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.

On Tue, 2011-02-22 at 17:39 -0800, Paul E. McKenney wrote:
> The build will break if you change the Kconfig to allow
> DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning
> near where the breakage would occur.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/rcupdate.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> index afd21d1..f3240e9 100644
> --- a/kernel/rcupdate.c
> +++ b/kernel/rcupdate.c
> @@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
>  		 * 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.
> +		 * Note that the machinery to reliably determine whether
> +		 * or not we are in an RCU read-side critical section
> +		 * exists only in the preemptible RCU implementations
> +		 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
> +		 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
>  		 */

Shouldn't this comment also be in the kconfig where
DEBUG_OBJECTS_RCU_HEAD is defined?

-- Steve

>  		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
>  		    irqs_disabled()) {


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