[PATCH v2] lib/spinlock_debug.c: prevent an infinite recursive cycle in spin_dump()

[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() -> spint_bug() -> spin_dump() -> printk()...
infinitely.

If the spin_bug() is called from a function like printk() which is
trying to obtain the console lock, we should prevent the debug spinlock
code from calling printk() again in that context.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/locking/spinlock_debug.c | 11 +++++++++++
 kernel/printk/printk.c          |  5 +++++
 2 files changed, 16 insertions(+)

diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..e745973 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -67,11 +67,22 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
 	dump_stack();
 }
 
+extern int is_console_lock(raw_spinlock_t *lock);
+
 static void spin_bug(raw_spinlock_t *lock, const char *msg)
 {
 	if (!debug_locks_off())
 		return;
 
+	/*
+	 * If this function is called from a function like printk()
+	 * which is trying to obtain the console lock, then we should
+	 * not call printk() any more. Or it will cause an infinite
+	 * recursive cycle!
+	 */
+	if (unlikely(is_console_lock(lock)))
+		return;
+
 	spin_dump(lock, msg);
 }
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2ce8826..50ea552 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -119,6 +119,11 @@ static int __down_trylock_console_sem(unsigned long ip)
 	up(&console_sem);\
 } while (0)
 
+int is_console_lock(raw_spinlock_t *lock)
+{
+	return &console_sem.lock == lock;
+}
+
 /*
  * This is used for debugging the mess that is the VT code by
  * keeping track if we have the console semaphore held. It's
-- 
1.9.1