md/raid5: need_this_block: start simplifying the last two conditions.

Both the last two cases are only relevant if something has failed and
something needs to be written (but not over-written), and if it is OK
to pre-read blocks at this point.  So factor out those tests and
explain them.

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

1 files changed, 23 insertions, 5 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 21c1e796a0d6..bb42551c1a42 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2930,16 +2930,34 @@ static int need_this_block(struct stripe_head *sh, struct stripe_head_state *s,
                  */
                 return 1;
 
+        /* Sometimes neither read-modify-write nor reconstruct-write
+         * cycles can work.  In those cases we read every block we
+         * can.  Then the parity-update is certain to have enough to
+         * work with.
+         * This can only be a problem when we need to write something,
+         * and some device has failed.  If either of those tests
+         * fail we need look no further.
+         */
+        if (!s->failed || !s->to_write)
+                return 0;
+
+        if (test_bit(R5_Insync, &dev->flags) &&
+            !test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
+                /* Pre-reads at not permitted until after short delay
+                 * to gather multiple requests.  However if this
+                 * device is no Insync, the block could only be be computed
+                 * and there is no need to delay that.
+                 */
+                return 0;
         if (
-             (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
-              (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) &&
+             (sh->raid_conf->level <= 5 && fdev[0]->towrite &&
               !test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||
              ((sh->raid_conf->level == 6 ||
                sh->sector >= sh->raid_conf->mddev->recovery_cp)
-              && s->failed && s->to_write &&
+              &&
               (s->to_write - s->non_overwrite <
-               sh->raid_conf->raid_disks - sh->raid_conf->max_degraded) &&
-              (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))))
+               sh->raid_conf->raid_disks - sh->raid_conf->max_degraded)
+              ))
                 return 1;
         return 0;
 }