[tip:sched/urgent] sched: Teach might_sleep() about preemptible RCU

[tip-bot for Frederic Weisbecker]

Commit-ID:  ba1b1cbcc9b458c4b9ebb28c9cf6a2ccf64ba1cd
Gitweb:     http://git.kernel.org/tip/ba1b1cbcc9b458c4b9ebb28c9cf6a2ccf64ba1cd
Author:     Frederic Weisbecker <fweisbec@xxxxxxxxx>
AuthorDate: Mon, 14 Dec 2009 23:44:32 +0100
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Wed, 16 Dec 2009 08:43:16 +0100

sched: Teach might_sleep() about preemptible 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 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.

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

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
LKML-Reference: <1260830672-7166-1-git-send-regression-fweisbec@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 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 db5c266..d76c790 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9655,7 +9655,7 @@ 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);
 }
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