md: improve the is_mddev_idle test fix

Don't use 'unsigned' variable to track sync vs non-sync IO, as the only thing
we want to do with them is a signed comparison, and fix up the comment which
had become quite wrong.

Signed-off-by: Neil Brown <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/md/md.c

···
 	mdk_rdev_t * rdev;
 	struct list_head *tmp;
 	int idle;
-	unsigned long curr_events;
+	long curr_events;
 
 	idle = 1;
 	ITERATE_RDEV(mddev,rdev,tmp) {
···
 		curr_events = disk_stat_read(disk, sectors[0]) + 
 				disk_stat_read(disk, sectors[1]) - 
 				atomic_read(&disk->sync_io);
-		/* The difference between curr_events and last_events
-		 * will be affected by any new non-sync IO (making
-		 * curr_events bigger) and any difference in the amount of
-		 * in-flight syncio (making current_events bigger or smaller)
-		 * The amount in-flight is currently limited to
-		 * 32*64K in raid1/10 and 256*PAGE_SIZE in raid5/6
-		 * which is at most 4096 sectors.
-		 * These numbers are fairly fragile and should be made
-		 * more robust, probably by enforcing the
-		 * 'window size' that md_do_sync sort-of uses.
+		/* sync IO will cause sync_io to increase before the disk_stats
+		 * as sync_io is counted when a request starts, and
+		 * disk_stats is counted when it completes.
+		 * So resync activity will cause curr_events to be smaller than
+		 * when there was no such activity.
+		 * non-sync IO will cause disk_stat to increase without
+		 * increasing sync_io so curr_events will (eventually)
+		 * be larger than it was before.  Once it becomes
+		 * substantially larger, the test below will cause
+		 * the array to appear non-idle, and resync will slow
+		 * down.
+		 * If there is a lot of outstanding resync activity when
+		 * we set last_event to curr_events, then all that activity
+		 * completing might cause the array to appear non-idle
+		 * and resync will be slowed down even though there might
+		 * not have been non-resync activity.  This will only
+		 * happen once though.  'last_events' will soon reflect
+		 * the state where there is little or no outstanding
+		 * resync requests, and further resync activity will
+		 * always make curr_events less than last_events.
 		 *
-		 * Note: the following is an unsigned comparison.
 		 */
-		if ((long)curr_events - (long)rdev->last_events > 4096) {
+		if (curr_events - rdev->last_events > 4096) {
 			rdev->last_events = curr_events;
 			idle = 0;
 		}

include/linux/raid/md_k.h

···
 
 	sector_t size;			/* Device size (in blocks) */
 	mddev_t *mddev;			/* RAID array if running */
-	unsigned long last_events;	/* IO event timestamp */
+	long last_events;		/* IO event timestamp */
 
 	struct block_device *bdev;	/* block device handle */