Re: rcu warnings cause stack overflow

[Heiko Carstens <heiko.carstens@de.ibm.com>]

On Thu, Feb 02, 2012 at 11:11:16AM -0800, Paul E. McKenney wrote:
> On Thu, Feb 02, 2012 at 03:52:20PM +0100, Frederic Weisbecker wrote:
> > On Thu, Feb 02, 2012 at 01:27:42PM +0100, Heiko Carstens wrote:
> > > On Wed, Feb 01, 2012 at 04:14:48PM +0100, Frederic Weisbecker wrote:
> > > > > Removing the WARN_ON_ONCE will fix this and, if lockdep is turned on, still
> > > > > will find illegal uses. But it won't work for lockdep off configs...
> > > > > So we probably want something better than the patch below.
> > > > 
> > > > Ah ok. Hmm, but why are you using an exception to implement WARN_ON()
> > > > in s390? Is it to have a whole new stack for the warning path in order
> > > > to avoid stack overflow from the place that called the WARN_ON() ?
> > > 
> > > The reason was to reduce the code footprint of the WARN_ON() and also
> > > be able to print the register contents at the time the warning happened.
> > 
> > Ah ok, makes sense.
> 
> So Frederic should push his anti-recursion patch, then?

Yes, please.

Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>

It still generates recursive warnings because the WARNON_ONCE is inlined and
every different usage will generate an exception, but it didn't produce a
stack overflow anymore.
To avoid the recursive warning the patch below would help. Not sure if it's
worth it...

Subject: [PATCH] rcu: move rcu_is_cpu_idle() check warning into C file

From: Heiko Carstens <heiko.carstens@de.ibm.com>

rcu_read_lock() and rcu_read_unlock() generate a warning if a cpu is in
extended quiescant state. Since these functions are inlined this can cause
a lot of warnings if in the processing of the WARN_ON_ONCE() there is
another usage of e.g. rcu_read_lock(). To make sure we only get one
warning (and avoid possible stack overflows) uninline the check.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---
 include/linux/rcupdate.h |    9 +++++++--
 kernel/rcupdate.c        |    6 ++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 81c04f4..9fe7be5 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -230,22 +230,27 @@ static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
 
 #ifdef CONFIG_PROVE_RCU
 extern int rcu_is_cpu_idle(void);
+extern void rcu_warn_if_is_cpu_idle(void);
 #else /* !CONFIG_PROVE_RCU */
 static inline int rcu_is_cpu_idle(void)
 {
 	return 0;
 }
+
+static inline void rcu_warn_if_is_cpu_idle(void)
+{
+}
 #endif /* else !CONFIG_PROVE_RCU */
 
 static inline void rcu_lock_acquire(struct lockdep_map *map)
 {
-	WARN_ON_ONCE(rcu_is_cpu_idle());
+	rcu_warn_if_is_cpu_idle();
 	lock_acquire(map, 0, 0, 2, 1, NULL, _THIS_IP_);
 }
 
 static inline void rcu_lock_release(struct lockdep_map *map)
 {
-	WARN_ON_ONCE(rcu_is_cpu_idle());
+	rcu_warn_if_is_cpu_idle();
 	lock_release(map, 1, _THIS_IP_);
 }
 
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 2bc4e13..5deca18 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -141,6 +141,12 @@ int rcu_my_thread_group_empty(void)
 	return thread_group_empty(current);
 }
 EXPORT_SYMBOL_GPL(rcu_my_thread_group_empty);
+
+void rcu_warn_if_is_cpu_idle(void)
+{
+	WARN_ON_ONCE(rcu_is_cpu_idle());
+}
+EXPORT_SYMBOL_GPL(rcu_warn_if_is_cpu_idle);
 #endif /* #ifdef CONFIG_PROVE_RCU */
 
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD