From: =?UTF-8?q?Uwe=20Kleine-K=C3=B6nig?= <redacted>

The patch "list_bl.h: make list head locking RT safe" introduced
an unconditional

	__set_bit(0, (unsigned long *)b);

in void hlist_bl_lock(struct hlist_bl_head *b). This clobbers the value
of b->first. When the value of b->first is retrieved using
hlist_bl_first the clobbering is undone using

	(unsigned long)h->first & ~LIST_BL_LOCKMASK

and so depending on LIST_BL_LOCKMASK being one. But LIST_BL_LOCKMASK is
only one if at least on of CONFIG_SMP and CONFIG_DEBUG_SPINLOCK are
defined. Without these the value returned by hlist_bl_first has the
zeroth bit set which likely results in a crash.

So only do the clobbering in the cases where LIST_BL_LOCKMASK is one.
An alternative would be to always define LIST_BL_LOCKMASK to one with
CONFIG_PREEMPT_RT_BASE.

Cc: stable-rt@vger.kernel.org
Acked-by: Paul Gortmaker <redacted>
Tested-by: Paul Gortmaker <redacted>
Signed-off-by: Uwe Kleine-König <redacted>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/list_bl.h |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h
index ddfd46a..becd7a6 100644
--- a/include/linux/list_bl.h
+++ b/include/linux/list_bl.h

@@ -131,8 +131,10 @@ static inline void hlist_bl_lock(struct hlist_bl_head *b)
 	bit_spin_lock(0, (unsigned long *)b);
 #else
 	raw_spin_lock(&b->lock);
+#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
 	__set_bit(0, (unsigned long *)b);
 #endif
+#endif
 }
 
 static inline void hlist_bl_unlock(struct hlist_bl_head *b)

@@ -140,7 +142,9 @@ static inline void hlist_bl_unlock(struct hlist_bl_head *b)
 #ifndef CONFIG_PREEMPT_RT_BASE
 	__bit_spin_unlock(0, (unsigned long *)b);
 #else
+#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
 	__clear_bit(0, (unsigned long *)b);
+#endif
 	raw_spin_unlock(&b->lock);
 #endif
 }

-- 
1.7.10.4