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

[Byungchul Park]

changes form v3 to v4
- reuse a existing code as much as possible for preventing an infinite
  recursive cycle.

changes from v2 to v3
- avoid printk() only in case of lockup suspected, not real lockup in
  which case it does not help at all.
- consider not only console_sem.lock but also logbuf_lock which is used
  by printk().

changes from v1 to v2
- only change comment and commit message esp. replacing "deadlock" with
  "infinite recursive cycle", since it is not an actual deadlock.

thanks,
byungchul

-----8<-----
>From 7b0c6e48625632fa1732b619083dc550b5d924c6 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.p...@lge.com>
Date: Wed, 27 Jan 2016 18:11:55 +0900
Subject: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the
 debug code

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.p...@lge.com>
---
 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