timer: Permit statically-declared work with deferrable timers

Currently, you have to just define a delayed_work uninitialised, and then
initialise it before first use. That's a tad clumsy. At risk of playing
mind-games with the compiler, fooling it into doing pointer arithmetic
with compile-time-constants, this lets clients properly initialise delayed
work with deferrable timers statically.

This patch was inspired by the issues which lead Artem Bityutskiy to
commit 8eab945c5616fc984 ("sunrpc: make the cache cleaner workqueue
deferrable").

Signed-off-by: Phil Carmody <[e-mail]>
Acked-by: Artem Bityutskiy <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

include/linux/timer.h

···
 #define __TIMER_LOCKDEP_MAP_INITIALIZER(_kn)
 #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.
+ *
+ * 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.
+ */
+#define TBASE_DEFERRABLE_FLAG		(0x1)
+
 #define TIMER_INITIALIZER(_function, _expires, _data) {		\
 		.entry = { .prev = TIMER_ENTRY_STATIC },	\
 		.function = (_function),			\
···
 		.data = (_data),				\
 		.base = &boot_tvec_bases,			\
 		.slack = -1,					\
+		__TIMER_LOCKDEP_MAP_INITIALIZER(		\
+			__FILE__ ":" __stringify(__LINE__))	\
+	}
+
+#define TBASE_MAKE_DEFERRED(ptr) ((struct tvec_base *)		\
+		  ((unsigned char *)(ptr) + TBASE_DEFERRABLE_FLAG))
+
+#define TIMER_DEFERRED_INITIALIZER(_function, _expires, _data) {\
+		.entry = { .prev = TIMER_ENTRY_STATIC },	\
+		.function = (_function),			\
+		.expires = (_expires),				\
+		.data = (_data),				\
+		.base = TBASE_MAKE_DEFERRED(&boot_tvec_bases),	\
 		__TIMER_LOCKDEP_MAP_INITIALIZER(		\
 			__FILE__ ":" __stringify(__LINE__))	\
 	}

include/linux/workqueue.h

···
 	.timer = TIMER_INITIALIZER(NULL, 0, 0),			\
 	}
 
+#define __DEFERRED_WORK_INITIALIZER(n, f) {			\
+	.work = __WORK_INITIALIZER((n).work, (f)),		\
+	.timer = TIMER_DEFERRED_INITIALIZER(NULL, 0, 0),	\
+	}
+
 #define DECLARE_WORK(n, f)					\
 	struct work_struct n = __WORK_INITIALIZER(n, f)
 
 #define DECLARE_DELAYED_WORK(n, f)				\
 	struct delayed_work n = __DELAYED_WORK_INITIALIZER(n, f)
+
+#define DECLARE_DEFERRED_WORK(n, f)				\
+	struct delayed_work n = __DEFERRED_WORK_INITIALIZER(n, f)
 
 /*
  * initialize a work item's function pointer

kernel/timer.c

···
 EXPORT_SYMBOL(boot_tvec_bases);
 static DEFINE_PER_CPU(struct tvec_base *, tvec_bases) = &boot_tvec_bases;
 
-/*
- * 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.
- *
- * 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.
- */
-#define TBASE_DEFERRABLE_FLAG		(0x1)
-
 /* Functions below help us manage 'deferrable' flag */
 static inline unsigned int tbase_get_deferrable(struct tvec_base *base)
 {
···
 
 static inline void timer_set_deferrable(struct timer_list *timer)
 {
-	timer->base = ((struct tvec_base *)((unsigned long)(timer->base) |
-				       TBASE_DEFERRABLE_FLAG));
+	timer->base = TBASE_MAKE_DEFERRED(timer->base);
 }
 
 static inline void