[PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the debug code

[Byungchul Park]

It causes an infinite recursive cycle when using CONFIG_DEBUG_SPINLOCK,
in the spin_dump(). Backtrace prints printk() -> console_trylock() ->
do_raw_spin_lock() -> spin_dump() -> printk()... infinitely.

When the debug spinlock code is called from printk(), we should prevent
calling spin_dump() and the like, calling printk() again in that context.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 include/linux/debug_locks.h     |  4 ++++
 kernel/locking/spinlock_debug.c | 14 +++++++++++---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h
index 822c135..b0f977e 100644
--- a/include/linux/debug_locks.h
+++ b/include/linux/debug_locks.h
@@ -10,6 +10,10 @@ struct task_struct;
 extern int debug_locks;
 extern int debug_locks_silent;
 
+static inline void __debug_locks_on(void)
+{
+	debug_locks = 1;
+}
 
 static inline int __debug_locks_off(void)
 {
diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..65177ba 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -113,11 +113,19 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
 			return;
 		__delay(1);
 	}
-	/* lockup suspected: */
-	spin_dump(lock, "lockup suspected");
+
+	/*
+	 * We should prevent calling printk() further, since it would cause
+	 * an infinite recursive cycle if it's called from printk()!
+	 */
+	if (__debug_locks_off()) {
+		/* lockup suspected: */
+		spin_dump(lock, "lockup suspected");
 #ifdef CONFIG_SMP
-	trigger_all_cpu_backtrace();
+		trigger_all_cpu_backtrace();
 #endif
+		__debug_locks_on();
+	}
 
 	/*
 	 * The trylock above was causing a livelock.  Give the lower level arch
-- 
1.9.1