dm block manager: make block locking optional

The block manager's locking is useful for catching cycles that may
result from certain btree metadata corruption. But in general it serves
as a developer tool to catch bugs in code. Unless you're finding that
DM thin provisioning is hanging due to infinite loops within the block
manager's access to btree nodes you can safely disable this feature.

Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]> # do/while(0) macro fix
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/Kconfig

···
 	 as a cache, holding recently-read blocks in memory and performing
 	 delayed writes.
 
+config DM_DEBUG_BLOCK_MANAGER_LOCKING
+       bool "Block manager locking"
+       depends on DM_BUFIO
+       ---help---
+	 Block manager locking can catch various metadata corruption issues.
+
+	 If unsure, say N.
+
 config DM_DEBUG_BLOCK_STACK_TRACING
        bool "Keep stack trace of persistent data block lock holders"
-       depends on STACKTRACE_SUPPORT && DM_BUFIO
+       depends on STACKTRACE_SUPPORT && DM_DEBUG_BLOCK_MANAGER_LOCKING
        select STACKTRACE
        ---help---
 	 Enable this for messages that may help debug problems with the

drivers/md/persistent-data/dm-block-manager.c

···
 
 /*----------------------------------------------------------------*/
 
+#ifdef CONFIG_DM_DEBUG_BLOCK_MANAGER_LOCKING
+
 /*
  * This is a read/write semaphore with a couple of differences.
  *
···
 		      (unsigned long long) b);
 }
 
+#else  /* !CONFIG_DM_DEBUG_BLOCK_MANAGER_LOCKING */
+
+#define bl_init(x) do { } while (0)
+#define bl_down_read(x) 0
+#define bl_down_read_nonblock(x) 0
+#define bl_up_read(x) do { } while (0)
+#define bl_down_write(x) 0
+#define bl_up_write(x) do { } while (0)
+#define report_recursive_bug(x, y) do { } while (0)
+
+#endif /* CONFIG_DM_DEBUG_BLOCK_MANAGER_LOCKING */
+
 /*----------------------------------------------------------------*/
 
 /*
···
 
 struct buffer_aux {
 	struct dm_block_validator *validator;
-	struct block_lock lock;
 	int write_locked;
+
+#ifdef CONFIG_DM_DEBUG_BLOCK_MANAGER_LOCKING
+	struct block_lock lock;
+#endif
 };
 
 static void dm_block_manager_alloc_callback(struct dm_buffer *buf)