linux-kernel - [PATCH] sched: Teach might_sleep about preemptable rcu

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Date:	Mon, 14 Dec 2009 23:44:32 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] sched: Teach might_sleep about preemptable rcu

In practice, it is harmless to voluntarily sleep in a rcu_read_lock()
section if we are running under preempt rcu, but it is illegal because
if we build a kernel running non-preemptable rcu.

Currently, might_sleep() doesn't notice sleepable operations under
rcu_read_lock() sections if we are running under preemptable rcu
because preempt_count() is left untouched after rcu_read_lock() in
this case. But we want developers who test their changes under such
config to notice the "sleeping while atomic" issues.

Then we add rcu_read_lock_nesting to prempt_count() in might_sleep()
checks.

Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 include/linux/rcutree.h |   11 +++++++++++
 kernel/sched.c          |    2 +-
 2 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index c93eee5..8044b1b 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -45,6 +45,12 @@ extern void __rcu_read_unlock(void);
 extern void synchronize_rcu(void);
 extern void exit_rcu(void);
 
+/*
+ * Defined as macro as it is a very low level header
+ * included from areas that don't even know about current
+ */
+#define rcu_preempt_depth() (current->rcu_read_lock_nesting)
+
 #else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
 
 static inline void __rcu_read_lock(void)
@@ -63,6 +69,11 @@ static inline void exit_rcu(void)
 {
 }
 
+static inline int rcu_preempt_depth(void)
+{
+	return 0;
+}
+
 #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
 
 static inline void __rcu_read_lock_bh(void)
diff --git a/kernel/sched.c b/kernel/sched.c
index ab42754..586c82c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9658,7 +9658,7 @@ void __init sched_init(void)
 #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
 static inline int preempt_count_equals(int preempt_offset)
 {
-	int nested = preempt_count() & ~PREEMPT_ACTIVE;
+	int nested = (preempt_count() & ~PREEMPT_ACTIVE) + rcu_preempt_depth();
 
 	return (nested == PREEMPT_INATOMIC_BASE + preempt_offset);
 }
-- 
1.6.2.3

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