dm cache metadata: fix READ_LOCK macros and cleanup WRITE_LOCK macros

The READ_LOCK macro was incorrectly returning -EINVAL if
dm_bm_is_read_only() was true -- it will always be true once the cache
metadata transitions to read-only by dm_cache_metadata_set_read_only().

Wrap READ_LOCK and WRITE_LOCK multi-statement macros in do {} while(0).
Also, all accesses of the 'cmd' argument passed to these related macros
are now encapsulated in parenthesis.

A follow-up patch can be developed to eliminate the use of macros in
favor of pure C code.  Avoiding that now given that this needs to apply
to stable@.
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Fixes: d14fcf3d ("dm cache: make sure every metadata function checks fail_io")
Cc: stable@vger.kernel.org

drivers/md/dm-cache-metadata.c

@@ -867,39 +867,55 @@ static int blocks_are_unmapped_or_clean(struct dm_cache_metadata *cmd,
 	return 0;
 }
 
+static bool cmd_write_lock(struct dm_cache_metadata *cmd)
+{
+	down_write(&cmd->root_lock);
+	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm)) {
+		up_write(&cmd->root_lock);
+		return false;
+	}
+	return true;
+}
+
 #define WRITE_LOCK(cmd)				\
-	down_write(&cmd->root_lock); \
-	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm)) { \
-		up_write(&cmd->root_lock); \
+	do {					\
+		if (!cmd_write_lock((cmd)))	\
 			return -EINVAL;		\
-	}
+	} while(0)
 
 #define WRITE_LOCK_VOID(cmd)			\
-	down_write(&cmd->root_lock); \
-	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm)) { \
-		up_write(&cmd->root_lock); \
+	do {					\
+		if (!cmd_write_lock((cmd)))	\
 			return;			\
-	}
+	} while(0)
 
 #define WRITE_UNLOCK(cmd) \
-	up_write(&cmd->root_lock)
+	up_write(&(cmd)->root_lock)
+
+static bool cmd_read_lock(struct dm_cache_metadata *cmd)
+{
+	down_write(&cmd->root_lock);
+	if (cmd->fail_io) {
+		up_write(&cmd->root_lock);
+		return false;
+	}
+	return true;
+}
 
 #define READ_LOCK(cmd)				\
-	down_read(&cmd->root_lock); \
-	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm)) { \
-		up_read(&cmd->root_lock); \
+	do {					\
+		if (!cmd_read_lock((cmd)))	\
 			return -EINVAL;		\
-	}
+	} while(0)
 
 #define READ_LOCK_VOID(cmd)			\
-	down_read(&cmd->root_lock); \
-	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm)) { \
-		up_read(&cmd->root_lock); \
+	do {					\
+		if (!cmd_read_lock((cmd)))	\
 			return;			\
-	}
+	} while(0)
 
 #define READ_UNLOCK(cmd) \
-	up_read(&cmd->root_lock)
+	up_read(&(cmd)->root_lock)
 
 int dm_cache_resize(struct dm_cache_metadata *cmd, dm_cblock_t new_cache_size)
 {