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>

1 files changed, 36 insertions, 56 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index a0322db4d355..45f8324196ec 100644
--- 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)