md: Allow read error in a single drive raid1 to be passed up.

If a raid1 only has a single working device and gets a read error, 
we choose to simply return that error up to the filesystem (or whatever)
rather than failing the whole array.

However the codes doesn't quite do that.  We attempt a readbalance
which allocates the same drive, so we retry the read - indefinitely. 

Instead:  If read_balance in the error case chooses the same drive that just
failed, treat it as a failure and don't retry.

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

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 7b4f5f7155d8..01e3cffd03b8 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1640,7 +1640,8 @@ static void raid1d(mddev_t *mddev)
                         }
 
                         bio = r1_bio->bios[r1_bio->read_disk];
-                        if ((disk=read_balance(conf, r1_bio)) == -1) {
+                        if ((disk=read_balance(conf, r1_bio)) == -1 ||
+                            disk == r1_bio->read_disk) {
                                 printk(KERN_ALERT "raid1: %s: unrecoverable I/O"
                                        " read error for block %llu\n",
                                        bdevname(bio->bi_bdev,b),