lockdep/timers: Explain in detail the locking problems del_timer_sync() may cause

Twice I had to explain the output about why lockdep gives an error with locks in IRQ context and with del_timer_sync(). Might as well write it up and place it in the comments above the code in del_timer_sync(). Perhaps the next time this lockdep dump triggers people will understand the issues. It is a ticky issue and very subtle, explaining it in detail in the code may help others understand the issue when they stumble upon the bug again. Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <1297186794.23343.19.camel@gandalf.stny.rr.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>

lockdep/timers: Explain in detail the locking problems del_timer_sync() may cause
Twice I had to explain the output about why lockdep gives an error with locks in IRQ context and with del_timer_sync(). Might as well write it up and place it in the comments above the code in del_timer_sync(). Perhaps the next time this lockdep dump triggers people will understand the issues. It is a ticky issue and very subtle, explaining it in detail in the code may help others understand the issue when they stumble upon the bug again. Signed-off-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <1297186794.23343.19.camel@gandalf.stny.rr.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Steven Rostedt · Ingo Molnar
1 parent a3ec4a603f
Showing 1 changed file with 23 additions and 0 deletions Side-by-side Diff
kernel/timer.c
@@ -964,6 +964,25 @@
  * 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 @@
 #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);
...	...	@@ -964,6 +964,25 @@
964	964	* add_timer_on(). Upon exit the timer is not queued and the handler is
965	965	* not running on any CPU.
966	966	*
	967	+ * Note: You must not hold locks that are held in interrupt context
	968	+ * while calling this function. Even if the lock has nothing to do
	969	+ * with the timer in question. Here's why:
	970	+ *
	971	+ * CPU0 CPU1
	972	+ * ---- ----
	973	+ * <SOFTIRQ>
	974	+ * call_timer_fn();
	975	+ * base->running_timer = mytimer;
	976	+ * spin_lock_irq(somelock);
	977	+ * <IRQ>
	978	+ * spin_lock(somelock);
	979	+ * del_timer_sync(mytimer);
	980	+ * while (base->running_timer == mytimer);
	981	+ *
	982	+ * Now del_timer_sync() will never return and never release somelock.
	983	+ * The interrupt on the other CPU is waiting to grab somelock but
	984	+ * it has interrupted the softirq that CPU0 is waiting to finish.
	985	+ *
967	986	* The function returns whether it has deactivated a pending timer or not.
968	987	*/
969	988	int del_timer_sync(struct timer_list *timer)
...	...	@@ -971,6 +990,10 @@
971	990	#ifdef CONFIG_LOCKDEP
972	991	unsigned long flags;
973	992
	993	+ /*
	994	+ * If lockdep gives a backtrace here, please reference
	995	+ * the synchronization rules above.
	996	+ */
974	997	local_irq_save(flags);
975	998	lock_map_acquire(&timer->lockdep_map);
976	999	lock_map_release(&timer->lockdep_map);