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SubjectRe: lockdep rcu-preempt and synchronize_srcu() awareness
On Mon, 2010-02-08 at 14:18 -0500, Mathieu Desnoyers wrote:
> Hi,
> I just though about the following deadlock scenario involving rcu preempt and
> mutexes. I see that lockdep does not warn about it, and it actually triggers a
> deadlock on my box. It might be worth addressing for TREE_PREEMPT_RCU configs.
> CPU A:
> mutex_lock(&test_mutex);
> synchronize_rcu();
> mutex_unlock(&test_mutex);
> CPU B:
> rcu_read_lock();
> mutex_lock(&test_mutex);
> mutex_unlock(&test_mutex);
> rcu_read_unlock();
> But given that it's not legit to take a mutex from within a rcu read lock in
> non-preemptible configs, I guess it's not much of a real-life problem, but I
> think SRCU is also affected, because there is no lockdep annotation around
> synchronize_srcu().

Right, even if there were, the lockdep rcu_read_lock annotation is
check==1, lockdep needs significant work to properly deal with fully
recursive locks such as rcu_read_lock(), the read side of rwlock_t and

Both ego and myself have been poking at that at various times but never
followed through, I think the last series is:

Once we have lock_acquire(.check=2, .read=2) working properly, adding
the above annotation is trivial, basically add:

lock_acquire(&rcu_lock_map, 0, 0, 0, 2, NULL, _THIS_IP_);
lock_release(&rcu_lock_map, 0, _THIS_IP_);

To the various synchronize_*() primitives with the respective lock_map.

> So I think it would be good to mark rcu_read_lock/unlock as not permitting
> "might_sleep()" in non preemptable RCU configs, and having a look at lockdep
> SRCU support might be worthwhile.

commit 234da7bcdc7aaa935846534c3b726dbc79a9cdd5
Author: Frederic Weisbecker <>
Date: Wed Dec 16 20:21:05 2009 +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.

[ v2: Handle rcu-tiny ]
Signed-off-by: Frederic Weisbecker <>
Reviewed-by: Paul E. McKenney <>
Cc: Peter Zijlstra <>
LKML-Reference: <>
Signed-off-by: Ingo Molnar <>

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index c4ba9a7..96cc307 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -101,4 +101,9 @@ static inline void exit_rcu(void)

+static inline int rcu_preempt_depth(void)
+ return 0;
#endif /* __LINUX_RCUTINY_H */
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 af7dfa7..7be88a7 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9682,7 +9682,7 @@ void __init sched_init(void)
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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