Merge branch 'core/locking' into auto-latest

2 files changed, 25 insertions, 3 deletions

diff --git a/kernel/rtmutex-tester.c b/kernel/rtmutex-tester.c
index d5b543506cbc..5c9ccd380966 100644
--- a/kernel/rtmutex-tester.c
+++ b/kernel/rtmutex-tester.c
@@ -44,9 +44,8 @@ enum test_opcodes {
 	RTTEST_LOCKINTNOWAIT,	/* 6 Lock interruptible no wait in wakeup, data = lockindex */
 	RTTEST_LOCKCONT,	/* 7 Continue locking after the wakeup delay */
 	RTTEST_UNLOCK,		/* 8 Unlock, data = lockindex */
-	RTTEST_LOCKBKL,		/* 9 Was: Lock BKL */
-	RTTEST_UNLOCKBKL,	/* 10 Was: Unlock BKL */
-	RTTEST_SIGNAL,		/* 11 Signal other test thread, data = thread id */
+	/* 9, 10 - reserved for BKL commemoration */
+	RTTEST_SIGNAL = 11,	/* 11 Signal other test thread, data = thread id */
 	RTTEST_RESETEVENT = 98,	/* 98 Reset event counter */
 	RTTEST_RESET = 99,	/* 99 Reset all pending operations */
 };
diff --git a/kernel/timer.c b/kernel/timer.c
index c848cd8abe2a..4234b08c0bf1 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -964,6 +964,25 @@ EXPORT_SYMBOL(try_to_del_timer_sync);
  * add_timer_on(). Upon exit the timer is not queued and the handler is
  * not running on any CPU.
  *
+ * Note: You must not hold locks that are held in interrupt context
+ *   while calling this function. Even if the lock has nothing to do
+ *   with the timer in question.  Here's why:
+ *
+ *    CPU0                             CPU1
+ *    ----                             ----
+ *                                   <SOFTIRQ>
+ *                                   call_timer_fn();
+ *                                     base->running_timer = mytimer;
+ *  spin_lock_irq(somelock);
+ *                                     <IRQ>
+ *                                        spin_lock(somelock);
+ *  del_timer_sync(mytimer);
+ *   while (base->running_timer == mytimer);
+ *
+ * Now del_timer_sync() will never return and never release somelock.
+ * The interrupt on the other CPU is waiting to grab somelock but
+ * it has interrupted the softirq that CPU0 is waiting to finish.
+ *
  * The function returns whether it has deactivated a pending timer or not.
  */
 int del_timer_sync(struct timer_list *timer)
@@ -971,6 +990,10 @@ int del_timer_sync(struct timer_list *timer)
 #ifdef CONFIG_LOCKDEP
 	unsigned long flags;
 
+	/*
+	 * If lockdep gives a backtrace here, please reference
+	 * the synchronization rules above.
+	 */
 	local_irq_save(flags);
 	lock_map_acquire(&timer->lockdep_map);
 	lock_map_release(&timer->lockdep_map);