timer: Implement TIMER_IRQSAFE

Timer internals are protected with irq-safe locks but timer execution
isn't, so a timer being dequeued for execution and its execution
aren't atomic against IRQs.  This makes it impossible to wait for its
completion from IRQ handlers and difficult to shoot down a timer from
IRQ handlers.

This issue caused some issues for delayed_work interface.  Because
there's no way to reliably shoot down delayed_work->timer from IRQ
handlers, __cancel_delayed_work() can't share the logic to steal the
target delayed_work with cancel_delayed_work_sync(), and can only
steal delayed_works which are on queued on timer.  Similarly, the
pending mod_delayed_work() can't be used from IRQ handlers.

This patch adds a new timer flag TIMER_IRQSAFE, which makes the timer
to be executed without enabling IRQ after dequeueing such that its
dequeueing and execution are atomic against IRQ handlers.

This makes it safe to wait for the timer's completion from IRQ
handlers, for example, using del_timer_sync().  It can never be
executing on the local CPU and if executing on other CPUs it won't be
interrupted until done.

This will enable simplifying delayed_work cancel/mod interface.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: torvalds@linux-foundation.org
Cc: peterz@infradead.org
Link: http://lkml.kernel.org/r/1344449428-24962-5-git-send-email-tj@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 24 insertions, 11 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 8d185a1..706fe4c 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -95,6 +95,11 @@ static inline unsigned int tbase_get_deferrable(struct tvec_base *base)
 	return ((unsigned int)(unsigned long)base & TIMER_DEFERRABLE);
 }
 
+static inline unsigned int tbase_get_irqsafe(struct tvec_base *base)
+{
+	return ((unsigned int)(unsigned long)base & TIMER_IRQSAFE);
+}
+
 static inline struct tvec_base *tbase_get_base(struct tvec_base *base)
 {
 	return ((struct tvec_base *)((unsigned long)base & ~TIMER_FLAG_MASK));
@@ -1002,14 +1007,14 @@ EXPORT_SYMBOL(try_to_del_timer_sync);
  *
  * Synchronization rules: Callers must prevent restarting of the timer,
  * otherwise this function is meaningless. It must not be called from
- * interrupt contexts. The caller must not hold locks which would prevent
- * completion of the timer's handler. The timer's handler must not call
- * add_timer_on(). Upon exit the timer is not queued and the handler is
- * not running on any CPU.
+ * interrupt contexts unless the timer is an irqsafe one. The caller must
+ * not hold locks which would prevent completion of the timer's
+ * handler. The timer's handler must not call 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:
+ * Note: For !irqsafe timers, 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
  *    ----                             ----
@@ -1046,7 +1051,7 @@ int del_timer_sync(struct timer_list *timer)
 	 * don't use it in hardirq context, because it
 	 * could lead to deadlock.
 	 */
-	WARN_ON(in_irq());
+	WARN_ON(in_irq() && !tbase_get_irqsafe(timer->base));
 	for (;;) {
 		int ret = try_to_del_timer_sync(timer);
 		if (ret >= 0)
@@ -1153,19 +1158,27 @@ static inline void __run_timers(struct tvec_base *base)
 		while (!list_empty(head)) {
 			void (*fn)(unsigned long);
 			unsigned long data;
+			bool irqsafe;
 
 			timer = list_first_entry(head, struct timer_list,entry);
 			fn = timer->function;
 			data = timer->data;
+			irqsafe = tbase_get_irqsafe(timer->base);
 
 			timer_stats_account_timer(timer);
 
 			base->running_timer = timer;
 			detach_expired_timer(timer, base);
 
-			spin_unlock_irq(&base->lock);
-			call_timer_fn(timer, fn, data);
-			spin_lock_irq(&base->lock);
+			if (irqsafe) {
+				spin_unlock(&base->lock);
+				call_timer_fn(timer, fn, data);
+				spin_lock(&base->lock);
+			} else {
+				spin_unlock_irq(&base->lock);
+				call_timer_fn(timer, fn, data);
+				spin_lock_irq(&base->lock);
+			}
 		}
 	}
 	base->running_timer = NULL;