btrfs: Adjust loop in free_extent_buffer

The loop construct in free_extent_buffer was added in
242e18c7 ("Btrfs: reduce lock contention on extent buffer locks")
as means of reducing the times the eb lock is taken, the non-last ref
count is decremented and lock is released. As the special handling
of UNMAPPED extent buffers was removed now there is only one decrement
op which is happening for EXTENT_BUFFER_UNMAPPED case.

This commit modifies the loop condition so that in case of UNMAPPED
buffers the eb's lock is taken only if we are 100% sure the eb is going
to be freed by the current executor of the code. Additionally, remove
superfluous ref count ops in btrfs test.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -5074,7 +5074,9 @@ void free_extent_buffer(struct extent_buffer *eb)
 
 	while (1) {
 		refs = atomic_read(&eb->refs);
-		if (refs <= 3)
+		if ((!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs <= 3)
+		    || (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) &&
+			refs == 1))
 			break;
 		old = atomic_cmpxchg(&eb->refs, refs, refs - 1);
 		if (old == refs)

fs/btrfs/tests/btrfs-tests.c

@@ -177,8 +177,6 @@ void btrfs_free_dummy_root(struct btrfs_root *root)
 	if (root->node) {
 		/* One for allocate_extent_buffer */
 		free_extent_buffer(root->node);
-		/* One for get_exent_buffer */
-		free_extent_buffer(root->node);
 	}
 	kfree(root);
 }

fs/btrfs/tests/inode-tests.c

@@ -254,11 +254,6 @@ static noinline int test_btrfs_get_extent(u32 sectorsize, u32 nodesize)
 		goto out;
 	}
 
-	/*
-	 * We will just free a dummy node if it's ref count is 2 so we need an
-	 * extra ref so our searches don't accidentally release our page.
-	 */
-	extent_buffer_get(root->node);
 	btrfs_set_header_nritems(root->node, 0);
 	btrfs_set_header_level(root->node, 0);
 	ret = -EINVAL;
@@ -860,7 +855,6 @@ static int test_hole_first(u32 sectorsize, u32 nodesize)
 		goto out;
 	}
 
-	extent_buffer_get(root->node);
 	btrfs_set_header_nritems(root->node, 0);
 	btrfs_set_header_level(root->node, 0);
 	BTRFS_I(inode)->root = root;