md/raid5: fetch_block must fetch all the blocks handle_stripe_dirtying wants.

commit 108cef3a upstream.

It is critical that fetch_block() and handle_stripe_dirtying()
are consistent in their analysis of what needs to be loaded.
Otherwise raid5 can wait forever for a block that won't be loaded.

Currently when writing to a RAID5 that is resyncing, to a location
beyond the resync offset, handle_stripe_dirtying chooses a
reconstruct-write cycle, but fetch_block() assumes a
read-modify-write, and a lockup can happen.

So treat that case just like RAID6, just as we do in
handle_stripe_dirtying.  RAID6 always does reconstruct-write.

This bug was introduced when the behaviour of handle_stripe_dirtying
was changed in 3.7, so the patch is suitable for any kernel since,
though it will need careful merging for some versions.

Cc: stable@vger.kernel.org (v3.7+)
Fixes: a7854487Reported-by: Henry Cai <henryplusplus@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

drivers/md/raid5.c

@@ -2789,7 +2789,8 @@ static int fetch_block(struct stripe_head *sh, struct stripe_head_state *s,
 	     (s->failed >= 2 && fdev[1]->toread) ||
 	     (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
 	      !test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||
-	     (sh->raid_conf->level == 6 && s->failed && s->to_write))) {
+	     ((sh->raid_conf->level == 6 || sh->sector >= sh->raid_conf->mddev->recovery_cp)
+	      && s->failed && s->to_write))) {
 		/* we would like to get this block, possibly by computing it,
 		 * otherwise read it if the backing disk is insync
 		 */