btrfs: remove btrfs_dev_replace::read_locks

This member seems to be copied from the extent_buffer locking scheme and
is at least used to assert that the read lock/unlock is properly nested.
In some way. While the _inc/_dec are called inside the read lock
section, the asserts are both inside and outside, so the ordering is not
guaranteed and we can see read/inc/dec ordered in any way
(theoretically).

A missing call of btrfs_dev_replace_clear_lock_blocking could cause
unexpected read_locks count, so this at least looks like a valid
assertion, but this will become unnecessary with later updates.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -361,7 +361,6 @@ struct btrfs_dev_replace {
 
 	struct mutex lock_finishing_cancel_unmount;
 	rwlock_t lock;
-	atomic_t read_locks;
 	atomic_t blocking_readers;
 	wait_queue_head_t read_lock_wq;
 

fs/btrfs/dev-replace.c

@@ -964,13 +964,10 @@ int btrfs_dev_replace_is_ongoing(struct btrfs_dev_replace *dev_replace)
 void btrfs_dev_replace_read_lock(struct btrfs_dev_replace *dev_replace)
 {
 	read_lock(&dev_replace->lock);
-	atomic_inc(&dev_replace->read_locks);
 }
 
 void btrfs_dev_replace_read_unlock(struct btrfs_dev_replace *dev_replace)
 {
-	ASSERT(atomic_read(&dev_replace->read_locks) > 0);
-	atomic_dec(&dev_replace->read_locks);
 	read_unlock(&dev_replace->lock);
 }
 
@@ -997,7 +994,6 @@ void btrfs_dev_replace_set_lock_blocking(
 					struct btrfs_dev_replace *dev_replace)
 {
 	/* only set blocking for read lock */
-	ASSERT(atomic_read(&dev_replace->read_locks) > 0);
 	atomic_inc(&dev_replace->blocking_readers);
 	read_unlock(&dev_replace->lock);
 }
@@ -1007,7 +1003,6 @@ void btrfs_dev_replace_clear_lock_blocking(
 					struct btrfs_dev_replace *dev_replace)
 {
 	/* only set blocking for read lock */
-	ASSERT(atomic_read(&dev_replace->read_locks) > 0);
 	ASSERT(atomic_read(&dev_replace->blocking_readers) > 0);
 	read_lock(&dev_replace->lock);
 	/* Barrier implied by atomic_dec_and_test */

fs/btrfs/disk-io.c

@@ -2155,7 +2155,6 @@ static void btrfs_init_dev_replace_locks(struct btrfs_fs_info *fs_info)
 {
 	mutex_init(&fs_info->dev_replace.lock_finishing_cancel_unmount);
 	rwlock_init(&fs_info->dev_replace.lock);
-	atomic_set(&fs_info->dev_replace.read_locks, 0);
 	atomic_set(&fs_info->dev_replace.blocking_readers, 0);
 	init_waitqueue_head(&fs_info->replace_wait);
 	init_waitqueue_head(&fs_info->dev_replace.read_lock_wq);