[PATCH v5] 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 spin_dump() is called from printk(), we should prevent the
debug spinlock code from calling printk() again in that context. It's
reasonable to avoid printing "lockup suspected" which is just a warning
message but it would cause a real lockup definitely.

The scenario is,

cpu0
====
printk
  console_trylock
  console_unlock
    up_console_sem
      up
        raw_spin_lock_irqsave(&sem->lock, flags)
        __up
          wake_up_process
            try_to_wake_up
              raw_spin_lock_irqsave(&p->pi_lock)
                __spin_lock_debug
                  spin_dump <=== the problem point!
                    printk
                      console_trylock
                        raw_spin_lock_irqsave(&sem->lock, flags)

                        <=== DEADLOCK

cpu1
====
printk
  console_trylock
    raw_spin_lock_irqsave(&sem->lock, flags)
    __spin_lock_debug
      spin_dump
        printk
          ...

          <=== repeat the recursive cycle infinitely

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/locking/spinlock_debug.c | 16 +++++++++++++---
 kernel/printk/printk.c          |  5 +++++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..cf7bc96 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -103,6 +103,8 @@ static inline void debug_spin_unlock(raw_spinlock_t *lock)
 	lock->owner_cpu = -1;
 }
 
+extern int is_console_lock(raw_spinlock_t *lock);
+
 static void __spin_lock_debug(raw_spinlock_t *lock)
 {
 	u64 i;
@@ -113,11 +115,19 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
 			return;
 		__delay(1);
 	}
-	/* lockup suspected: */
-	spin_dump(lock, "lockup suspected");
+
+	/*
+	 * If this function is called from printk(), then we should
+	 * not call printk() more. Or it will cause an infinite
+	 * recursive cycle!
+	 */
+	if (likely(!is_console_lock(lock))) {
+		/* lockup suspected: */
+		spin_dump(lock, "lockup suspected");
 #ifdef CONFIG_SMP
-	trigger_all_cpu_backtrace();
+		trigger_all_cpu_backtrace();
 #endif
+	}
 
 	/*
 	 * The trylock above was causing a livelock.  Give the lower level arch
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2ce8826..568ab11 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1981,6 +1981,11 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
 }
 #endif
 
+int is_console_lock(raw_spinlock_t *lock)
+{
+	return lock == &console_sem.lock;
+}
+
 static int __add_preferred_console(char *name, int idx, char *options,
 				   char *brl_options)
 {
-- 
1.9.1