btrfs: kill btrfs_fs_info::volume_mutex

Mutual exclusion of device add/rm and balance was done by the volume
mutex up to version 3.7. The commit 5ac00addc7ac091109 ("Btrfs: disallow
mutually exclusive admin operations from user mode") added a bit that
essentially tracked the same information.

The status bit has an advantage over a mutex that it can be set without
restrictions of function context, so it started to be used in the
mount-time resuming of balance or device replace.

But we don't really need to track the same information in two ways.

1) After the previous cleanups, the main ioctl handlers for
   add/del/resize copy the EXCL_OP bit next to the volume mutex, here
   it's clearly safe.

2) Resuming balance during mount or after rw remount will set only the
   EXCL_OP bit and the volume_mutex is held in the kernel thread that
   calls btrfs_balance.

3) Resuming device replace during mount or after rw remount is done
   after balance and is excluded by the EXCL_OP bit. It does not take
   the volume_mutex at all and completely relies on the EXCL_OP bit.

4) The resuming of balance and dev-replace cannot hapen at the same time
   as the ioctls cannot be started in parallel. Nevertheless, a crafted
   image could trigger that and a warning is printed.

5) Balance is normally excluded by EXCL_OP and also uses own mutex to
   protect against concurrent access to its status data. There's some
   trickery to maintain the right lock nesting in case we need to
   reexamine the status in btrfs_ioctl_balance. The volume_mutex is
   removed and the unlock/lock sequence is left in place as we might
   expect other waiters to proceed.

6) Similar to 5, the unlock/lock sequence is kept in
   btrfs_cancel_balance to allow waiters to continue.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 7 insertions, 22 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 9e5d27dd00b7..0216bc86f476 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -167,12 +167,6 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
  * may be used to exclude some operations from running concurrently without any
  * modifications to the list (see write_all_supers)
  *
- * volume_mutex
- * ------------
- * coarse lock owned by a mounted filesystem; used to exclude some operations
- * that cannot run in parallel and affect the higher-level properties of the
- * filesystem like: device add/deleting/resize/replace, or balance
- *
  * balance_mutex
  * -------------
  * protects balance structures (status, state) and context accessed from
@@ -3206,9 +3200,8 @@ static void update_balance_args(struct btrfs_balance_control *bctl)
 }
 
 /*
- * Should be called with both balance and volume mutexes held to
- * serialize other volume operations (add_dev/rm_dev/resize) with
- * restriper.  Same goes for reset_balance_state.
+ * Should be called with balance mutex held to protect against checking the
+ * balance status or progress. Same goes for reset_balance_state.
  */
 static void set_balance_control(struct btrfs_balance_control *bctl)
 {
@@ -3785,7 +3778,7 @@ static inline int validate_convert_profile(struct btrfs_balance_args *bctl_arg,
 }
 
 /*
- * Should be called with both balance and volume mutexes held
+ * Should be called with balance mutexe held
  */
 int btrfs_balance(struct btrfs_balance_control *bctl,
 		  struct btrfs_ioctl_balance_args *bargs)
@@ -3951,16 +3944,12 @@ static int balance_kthread(void *data)
 	struct btrfs_fs_info *fs_info = data;
 	int ret = 0;
 
-	mutex_lock(&fs_info->volume_mutex);
 	mutex_lock(&fs_info->balance_mutex);
-
 	if (fs_info->balance_ctl) {
 		btrfs_info(fs_info, "continuing balance");
 		ret = btrfs_balance(fs_info->balance_ctl, NULL);
 	}
-
 	mutex_unlock(&fs_info->balance_mutex);
-	mutex_unlock(&fs_info->volume_mutex);
 
 	return ret;
 }
@@ -4054,13 +4043,9 @@ int btrfs_recover_balance(struct btrfs_fs_info *fs_info)
 		btrfs_warn(fs_info,
 	"cannot set exclusive op status to balance, resume manually");
 
-	mutex_lock(&fs_info->volume_mutex);
 	mutex_lock(&fs_info->balance_mutex);
-
 	set_balance_control(bctl);
-
 	mutex_unlock(&fs_info->balance_mutex);
-	mutex_unlock(&fs_info->volume_mutex);
 out:
 	btrfs_free_path(path);
 	return ret;
@@ -4117,17 +4102,17 @@ int btrfs_cancel_balance(struct btrfs_fs_info *fs_info)
 			   atomic_read(&fs_info->balance_running) == 0);
 		mutex_lock(&fs_info->balance_mutex);
 	} else {
-		/* reset_balance_state needs volume_mutex */
 		mutex_unlock(&fs_info->balance_mutex);
-		mutex_lock(&fs_info->volume_mutex);
+		/*
+		 * Lock released to allow other waiters to continue, we'll
+		 * reexamine the status again.
+		 */
 		mutex_lock(&fs_info->balance_mutex);
 
 		if (fs_info->balance_ctl) {
 			reset_balance_state(fs_info);
 			clear_bit(BTRFS_FS_EXCL_OP, &fs_info->flags);
 		}
-
-		mutex_unlock(&fs_info->volume_mutex);
 	}
 
 	BUG_ON(fs_info->balance_ctl || atomic_read(&fs_info->balance_running));