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>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 21c1e796a0d6ab9fe9c9b4489fd3b6d73808128d..bb42551c1a42cbb189b53c225c139bf62f2ef65e 100644 (file)
--- 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;
 }