md: tidy up device searches in read_balance.

The code for searching through the device list to read-balance in
raid1 is rather clumsy and hard to follow.  Try to simplify it a bit.

No important functionality change here.

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

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index a0322db4d355d111bc7ed8f34f820b2894b637bc..45f8324196ec61c86cd7164868e3f8bc66ada03c 100644 (file)
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -419,11 +419,13 @@ static void raid1_end_write_request(struct bio *bio, int error)
 static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 {
        const sector_t this_sector = r1_bio->sector;
-       int new_disk = conf->last_used, disk = new_disk;
-       int wonly_disk = -1;
        const int sectors = r1_bio->sectors;
+       int new_disk = -1;
+       int start_disk;
+       int i;
        sector_t new_distance, current_distance;
        mdk_rdev_t *rdev;
+       int choose_first;
 
        rcu_read_lock();
        /*
@@ -434,54 +436,33 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
  retry:
        if (conf->mddev->recovery_cp < MaxSector &&
            (this_sector + sectors >= conf->next_resync)) {
-               /* Choose the first operational device, for consistancy */
-               new_disk = 0;
-
-               for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
-                    r1_bio->bios[new_disk] == IO_BLOCKED ||
-                    !rdev || !test_bit(In_sync, &rdev->flags)
-                            || test_bit(WriteMostly, &rdev->flags);
-                    rdev = rcu_dereference(conf->mirrors[++new_disk].rdev)) {
-
-                       if (rdev && test_bit(In_sync, &rdev->flags) &&
-                               r1_bio->bios[new_disk] != IO_BLOCKED)
-                               wonly_disk = new_disk;
-
-                       if (new_disk == conf->raid_disks - 1) {
-                               new_disk = wonly_disk;
-                               break;
-                       }
-               }
-               goto rb_out;
+               choose_first = 1;
+               start_disk = 0;
+       } else {
+               choose_first = 0;
+               start_disk = conf->last_used;
        }
 
-
        /* make sure the disk is operational */
-       for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
-            r1_bio->bios[new_disk] == IO_BLOCKED ||
-            !rdev || !test_bit(In_sync, &rdev->flags) ||
-                    test_bit(WriteMostly, &rdev->flags);
-            rdev = rcu_dereference(conf->mirrors[new_disk].rdev)) {
-
-               if (rdev && test_bit(In_sync, &rdev->flags) &&
-                   r1_bio->bios[new_disk] != IO_BLOCKED)
-                       wonly_disk = new_disk;
-
-               if (new_disk <= 0)
-                       new_disk = conf->raid_disks;
-               new_disk--;
-               if (new_disk == disk) {
-                       new_disk = wonly_disk;
+       for (i = 0 ; i < conf->raid_disks ; i++) {
+               int disk = start_disk + i;
+               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))
+                       continue;
+
+               new_disk = disk;
+               if (!test_bit(WriteMostly, &rdev->flags))
                        break;
-               }
        }
 
-       if (new_disk < 0)
+       if (new_disk < 0 || choose_first)
                goto rb_out;
 
-       disk = new_disk;
-       /* now disk == new_disk == starting point for search */
-
        /*
         * Don't change to another disk for sequential reads:
         */
@@ -490,20 +471,21 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
        if (this_sector == conf->mirrors[new_disk].head_position)
                goto rb_out;
 
-       current_distance = abs(this_sector - conf->mirrors[disk].head_position);
+       current_distance = abs(this_sector 
+                              - conf->mirrors[new_disk].head_position);
 
-       /* Find the disk whose head is closest */
-
-       do {
-               if (disk <= 0)
-                       disk = conf->raid_disks;
-               disk--;
+       /* 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 (!rdev || r1_bio->bios[disk] == IO_BLOCKED ||
-                   !test_bit(In_sync, &rdev->flags) ||
-                   test_bit(WriteMostly, &rdev->flags))
+               if (r1_bio->bios[disk] == IO_BLOCKED
+                   || rdev == NULL
+                   || !test_bit(In_sync, &rdev->flags)
+                   || test_bit(WriteMostly, &rdev->flags))
                        continue;
 
                if (!atomic_read(&rdev->nr_pending)) {
@@ -515,11 +497,9 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
                        current_distance = new_distance;
                        new_disk = disk;
                }
-       } while (disk != conf->last_used);
+       }
 
  rb_out:
-
-
        if (new_disk >= 0) {
                rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
                if (!rdev)