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Commit 48228f7b authored by Steven Rostedt's avatar Steven Rostedt Committed by Ingo Molnar
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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: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1297186794.23343.19.camel@gandalf.stny.rr.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent a3ec4a60

kernel/timer.c

+23 −0
Original line number Diff line number Diff line
@@ -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);