workqueue: introduce WORK_OFFQ_FLAG_*

Low WORK_STRUCT_FLAG_BITS bits of work_struct->data contain
WORK_STRUCT_FLAG_* and flush color.  If the work item is queued, the
rest point to the cpu_workqueue with WORK_STRUCT_CWQ set; otherwise,
WORK_STRUCT_CWQ is clear and the bits contain the last CPU number -
either a real CPU number or one of WORK_CPU_*.

Scheduled addition of mod_delayed_work[_on]() requires an additional
flag, which is used only while a work item is off queue.  There are
more than enough bits to represent off-queue CPU number on both 32 and
64bits.  This patch introduces WORK_OFFQ_FLAG_* which occupy the lower
part of the @work->data high bits while off queue.  This patch doesn't
define any actual OFFQ flag yet.

Off-queue CPU number is now shifted by WORK_OFFQ_CPU_SHIFT, which adds
the number of bits used by OFFQ flags to WORK_STRUCT_FLAG_SHIFT, to
make room for OFFQ flags.

To avoid shift width warning with large WORK_OFFQ_FLAG_BITS, ulong
cast is added to WORK_STRUCT_NO_CPU and, just in case, BUILD_BUG_ON()
to check that there are enough bits to accomodate off-queue CPU number
is added.

This patch doesn't make any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -68,9 +68,15 @@ enum {
 	WORK_STRUCT_FLAG_BITS	= WORK_STRUCT_COLOR_SHIFT +
 				  WORK_STRUCT_COLOR_BITS,
 
+	/* data contains off-queue information when !WORK_STRUCT_CWQ */
+	WORK_OFFQ_FLAG_BASE	= WORK_STRUCT_FLAG_BITS,
+	WORK_OFFQ_FLAG_BITS	= 0,
+	WORK_OFFQ_CPU_SHIFT	= WORK_OFFQ_FLAG_BASE + WORK_OFFQ_FLAG_BITS,
+
+	/* convenience constants */
 	WORK_STRUCT_FLAG_MASK	= (1UL << WORK_STRUCT_FLAG_BITS) - 1,
 	WORK_STRUCT_WQ_DATA_MASK = ~WORK_STRUCT_FLAG_MASK,
-	WORK_STRUCT_NO_CPU	= WORK_CPU_NONE << WORK_STRUCT_FLAG_BITS,
+	WORK_STRUCT_NO_CPU	= (unsigned long)WORK_CPU_NONE << WORK_OFFQ_CPU_SHIFT,
 
 	/* bit mask for work_busy() return values */
 	WORK_BUSY_PENDING	= 1 << 0,

kernel/workqueue.c

@@ -533,9 +533,9 @@ static int work_next_color(int color)
 }
 
 /*
- * A work's data points to the cwq with WORK_STRUCT_CWQ set while the
- * work is on queue.  Once execution starts, WORK_STRUCT_CWQ is
- * cleared and the work data contains the cpu number it was last on.
+ * While queued, %WORK_STRUCT_CWQ is set and non flag bits of a work's data
+ * contain the pointer to the queued cwq.  Once execution starts, the flag
+ * is cleared and the high bits contain OFFQ flags and CPU number.
  *
  * set_work_cwq(), set_work_cpu_and_clear_pending() and clear_work_data()
  * can be used to set the cwq, cpu or clear work->data.  These functions
@@ -565,7 +565,7 @@ static void set_work_cwq(struct work_struct *work,
 static void set_work_cpu_and_clear_pending(struct work_struct *work,
 					   unsigned int cpu)
 {
-	set_work_data(work, cpu << WORK_STRUCT_FLAG_BITS, 0);
+	set_work_data(work, (unsigned long)cpu << WORK_OFFQ_CPU_SHIFT, 0);
 }
 
 static void clear_work_data(struct work_struct *work)
@@ -592,7 +592,7 @@ static struct global_cwq *get_work_gcwq(struct work_struct *work)
 		return ((struct cpu_workqueue_struct *)
 			(data & WORK_STRUCT_WQ_DATA_MASK))->pool->gcwq;
 
-	cpu = data >> WORK_STRUCT_FLAG_BITS;
+	cpu = data >> WORK_OFFQ_CPU_SHIFT;
 	if (cpu == WORK_CPU_NONE)
 		return NULL;
 
@@ -3724,6 +3724,10 @@ static int __init init_workqueues(void)
 	unsigned int cpu;
 	int i;
 
+	/* make sure we have enough bits for OFFQ CPU number */
+	BUILD_BUG_ON((1LU << (BITS_PER_LONG - WORK_OFFQ_CPU_SHIFT)) <
+		     WORK_CPU_LAST);
+
 	cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
 	cpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);