dm raid1: clear region outside spinlock

A clear_region function is permitted to block (in practice, rare) but gets
called in rh_update_states() with a spinlock held.

The bits being marked and cleared by the above functions are used
to update the on-disk log, but are never read directly.  We can
perform these operations outside the spinlock since the
bits are only changed within one thread viz.
   - mark_region in rh_inc()
   - clear_region in rh_update_states().

So, we grab the clean_regions list items via list_splice() within the
spinlock and defer clear_region() until we iterate over the list for
deletion - similar to how the recovered_regions list is already handled.
We then move the flush() call down to ensure it encapsulates the changes
which are done by the later calls to clear_region().

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/dm-raid1.c

@@ -378,10 +378,8 @@ static void rh_update_states(struct region_hash *rh)
 		list_splice(&rh->clean_regions, &clean);
 		INIT_LIST_HEAD(&rh->clean_regions);
 
-		list_for_each_entry (reg, &clean, list) {
-			rh->log->type->clear_region(rh->log, reg->key);
+		list_for_each_entry(reg, &clean, list)
 			list_del(&reg->hash_list);
-		}
 	}
 
 	if (!list_empty(&rh->recovered_regions)) {
@@ -405,10 +403,12 @@ static void rh_update_states(struct region_hash *rh)
 		mempool_free(reg, rh->region_pool);
 	}
 
-	rh->log->type->flush(rh->log);
-
-	list_for_each_entry_safe (reg, next, &clean, list)
+	list_for_each_entry_safe(reg, next, &clean, list) {
+		rh->log->type->clear_region(rh->log, reg->key);
 		mempool_free(reg, rh->region_pool);
+	}
+
+	rh->log->type->flush(rh->log);
 }
 
 static void rh_inc(struct region_hash *rh, region_t region)