dm cache: fix a harmless race when working out if a block is discarded

It is more correct to hold the cell before checking the discard state.
These flags are only used as hints to the policy so this change will
have negligable effect.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -1432,9 +1432,8 @@ static void process_bio(struct cache *cache, struct prealloc *structs,
 	dm_oblock_t block = get_bio_block(cache, bio);
 	struct dm_bio_prison_cell *cell_prealloc, *old_ocell, *new_ocell;
 	struct policy_result lookup_result;
-	bool discarded_block = is_discarded_oblock(cache, block);
 	bool passthrough = passthrough_mode(&cache->features);
-	bool can_migrate = !passthrough && (discarded_block || spare_migration_bandwidth(cache));
+	bool discarded_block, can_migrate;
 
 	/*
 	 * Check to see if that block is currently migrating.
@@ -1446,6 +1445,9 @@ static void process_bio(struct cache *cache, struct prealloc *structs,
 	if (r > 0)
 		return;
 
+	discarded_block = is_discarded_oblock(cache, block);
+	can_migrate = !passthrough && (discarded_block || spare_migration_bandwidth(cache));
+
 	r = policy_map(cache->policy, block, true, can_migrate, discarded_block,
 		       bio, &lookup_result);