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, 12 insertions, 4 deletions

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 3f95c1fa615e..8c5a197e1587 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -49,18 +49,26 @@ extern struct tvec_base boot_tvec_bases;
 #endif
 
 /*
- * Note that all tvec_bases are 2 byte aligned and lower bit of
- * base in timer_list is guaranteed to be zero. Use the LSB to
- * indicate whether the timer is deferrable.
+ * Note that all tvec_bases are at least 4 byte aligned and lower two bits
+ * of base in timer_list is guaranteed to be zero. Use them for flags.
  *
  * A deferrable timer will work normally when the system is busy, but
  * will not cause a CPU to come out of idle just to service it; instead,
  * the timer will be serviced when the CPU eventually wakes up with a
  * subsequent non-deferrable timer.
+ *
+ * An irqsafe timer is executed with IRQ disabled and it's safe to wait for
+ * the completion of the running instance from IRQ handlers, for example,
+ * by calling del_timer_sync().
+ *
+ * Note: The irq disabled callback execution is a special case for
+ * workqueue locking issues. It's not meant for executing random crap
+ * with interrupts disabled. Abuse is monitored!
  */
 #define TIMER_DEFERRABLE		0x1LU
+#define TIMER_IRQSAFE			0x2LU
 
-#define TIMER_FLAG_MASK			0x1LU
+#define TIMER_FLAG_MASK			0x3LU
 
 #define __TIMER_INITIALIZER(_function, _expires, _data, _flags) { \
 		.entry = { .prev = TIMER_ENTRY_STATIC },	\