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author | Steven Rostedt <rostedt@goodmis.org> | 2011-02-08 12:39:54 -0500 |
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committer | Ingo Molnar <mingo@elte.hu> | 2011-02-16 13:35:08 +0100 |
commit | 48228f7b470a74b6469a250d2977a13128d8fe96 (patch) | |
tree | 1ed8f82ffef779ccf665f68c1fcf302d34d09826 | |
parent | a3ec4a603faf4244e275bf11b467aad092dfbd8a (diff) | |
download | kernel_samsung_espresso10-48228f7b470a74b6469a250d2977a13128d8fe96.zip kernel_samsung_espresso10-48228f7b470a74b6469a250d2977a13128d8fe96.tar.gz kernel_samsung_espresso10-48228f7b470a74b6469a250d2977a13128d8fe96.tar.bz2 |
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>
-rw-r--r-- | kernel/timer.c | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/kernel/timer.c b/kernel/timer.c index d645992..5f40c2e 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); |