md/raid1: clean up read_balance.

read_balance has two loops which both look for a 'best'
device based on slightly different criteria.
This is clumsy and makes is hard to add extra criteria.

So replace it all with a single loop that combines everything.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 34 insertions, 49 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 2b7a7ff401d..f0b0c79b389 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -411,10 +411,10 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 {
         const sector_t this_sector = r1_bio->sector;
         const int sectors = r1_bio->sectors;
-        int new_disk = -1;
         int start_disk;
+        int best_disk;
         int i;
-        sector_t new_distance, current_distance;
+        sector_t best_dist;
         mdk_rdev_t *rdev;
         int choose_first;
 
@@ -425,6 +425,8 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
          * We take the first readable disk when above the resync window.
          */
  retry:
+        best_disk = -1;
+        best_dist = MaxSector;
         if (conf->mddev->recovery_cp < MaxSector &&
             (this_sector + sectors >= conf->next_resync)) {
                 choose_first = 1;
@@ -434,8 +436,8 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
                 start_disk = conf->last_used;
         }
 
-        /* make sure the disk is operational */
         for (i = 0 ; i < conf->raid_disks ; i++) {
+                sector_t dist;
                 int disk = start_disk + i;
                 if (disk >= conf->raid_disks)
                         disk -= conf->raid_disks;
@@ -443,60 +445,43 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
                 rdev = rcu_dereference(conf->mirrors[disk].rdev);
                 if (r1_bio->bios[disk] == IO_BLOCKED
                     || rdev == NULL
-                    || !test_bit(In_sync, &rdev->flags))
+                    || test_bit(Faulty, &rdev->flags))
                         continue;
-
-                new_disk = disk;
-                if (!test_bit(WriteMostly, &rdev->flags))
-                        break;
-        }
-
-        if (new_disk < 0 || choose_first)
-                goto rb_out;
-
-        /*
-         * Don't change to another disk for sequential reads:
-         */
-        if (conf->next_seq_sect == this_sector)
-                goto rb_out;
-        if (this_sector == conf->mirrors[new_disk].head_position)
-                goto rb_out;
-
-        current_distance = abs(this_sector 
-                               - conf->mirrors[new_disk].head_position);
-
-        /* look for a better disk - i.e. head is closer */
-        start_disk = new_disk;
-        for (i = 1; i < conf->raid_disks; i++) {
-                int disk = start_disk + 1;
-                if (disk >= conf->raid_disks)
-                        disk -= conf->raid_disks;
-
-                rdev = rcu_dereference(conf->mirrors[disk].rdev);
-                if (r1_bio->bios[disk] == IO_BLOCKED
-                    || rdev == NULL
-                    || !test_bit(In_sync, &rdev->flags)
-                    || test_bit(WriteMostly, &rdev->flags))
+                if (!test_bit(In_sync, &rdev->flags) &&
+                    rdev->recovery_offset < this_sector + sectors)
                         continue;
-
-                if (!atomic_read(&rdev->nr_pending)) {
-                        new_disk = disk;
+                if (test_bit(WriteMostly, &rdev->flags)) {
+                        /* Don't balance among write-mostly, just
+                         * use the first as a last resort */
+                        if (best_disk < 0)
+                                best_disk = disk;
+                        continue;
+                }
+                /* This is a reasonable device to use.  It might
+                 * even be best.
+                 */
+                dist = abs(this_sector - conf->mirrors[disk].head_position);
+                if (choose_first
+                    /* Don't change to another disk for sequential reads */
+                    || conf->next_seq_sect == this_sector
+                    || dist == 0
+                    /* If device is idle, use it */
+                    || atomic_read(&rdev->nr_pending) == 0) {
+                        best_disk = disk;
                         break;
                 }
-                new_distance = abs(this_sector - conf->mirrors[disk].head_position);
-                if (new_distance < current_distance) {
-                        current_distance = new_distance;
-                        new_disk = disk;
+                if (dist < best_dist) {
+                        best_dist = dist;
+                        best_disk = disk;
                 }
         }
 
- rb_out:
-        if (new_disk >= 0) {
-                rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
+        if (best_disk >= 0) {
+                rdev = rcu_dereference(conf->mirrors[best_disk].rdev);
                 if (!rdev)
                         goto retry;
                 atomic_inc(&rdev->nr_pending);
-                if (!test_bit(In_sync, &rdev->flags)) {
+                if (test_bit(Faulty, &rdev->flags)) {
                         /* cannot risk returning a device that failed
                          * before we inc'ed nr_pending
                          */
@@ -504,11 +489,11 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
                         goto retry;
                 }
                 conf->next_seq_sect = this_sector + sectors;
-                conf->last_used = new_disk;
+                conf->last_used = best_disk;
         }
         rcu_read_unlock();
 
-        return new_disk;
+        return best_disk;
 }
 
 static int raid1_congested(void *data, int bits)