1 files changed, 23 insertions, 0 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 33a67925d90..3503c17ac1d 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -970,6 +970,25 @@ 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)
@@ -977,6 +996,10 @@ 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);