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

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Message-Id: <1298425183-21265-5-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue, 22 Feb 2011 17:39:34 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	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, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.

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.
 		 */
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
-- 
1.7.3.2

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