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Subject[tip:core/locking] lockdep/timers: Explain in detail the locking problems del_timer_sync() may cause
Commit-ID:  48228f7b470a74b6469a250d2977a13128d8fe96
Gitweb: http://git.kernel.org/tip/48228f7b470a74b6469a250d2977a13128d8fe96
Author: Steven Rostedt <rostedt@goodmis.org>
AuthorDate: Tue, 8 Feb 2011 12:39:54 -0500
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 16 Feb 2011 13:35:08 +0100

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>
---
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);

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