fs/btrfs/qgroup.c:1616:18: warning: array subscript is below array bounds

fs/btrfs/qgroup.c:1616:18: warning: array subscript is below array bounds

[kbuild test robot]

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tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   b4b8664d291ac1998e0f0bcdc96b6397f0fe68b3
commit: 33d1f05ccb698aa92db3e64a639ce523cf18a408 btrfs: Export and move leaf/subtree qgroup helpers to qgroup.c
date:   4 weeks ago
config: x86_64-randconfig-in0-12270518 (attached as .config)
compiler: gcc-4.6 (Debian 4.6.4-7) 4.6.4
reproduce:
        git checkout 33d1f05ccb698aa92db3e64a639ce523cf18a408
        # save the attached .config to linux build tree
        make ARCH=x86_64 

Note: it may well be a FALSE warning. FWIW you are at least aware of it now.

All warnings (new ones prefixed by >>):

   fs/btrfs/qgroup.c: In function 'adjust_slots_upwards.isra.13':
>> fs/btrfs/qgroup.c:1616:18: warning: array subscript is below array bounds [-Warray-bounds]
   fs/btrfs/qgroup.c:1617:2: warning: array subscript is below array bounds [-Warray-bounds]

vim +1616 fs/btrfs/qgroup.c

  1600					free_extent_buffer(eb);
  1601					path->nodes[level] = NULL;
  1602					path->slots[level] = 0;
  1603				}
  1604			} else {
  1605				/*
  1606				 * We have a valid slot to walk back down
  1607				 * from. Stop here so caller can process these
  1608				 * new nodes.
  1609				 */
  1610				break;
  1611			}
  1612	
  1613			level++;
  1614		}
  1615	
> 1616		eb = path->nodes[root_level];
  1617		if (path->slots[root_level] >= btrfs_header_nritems(eb))
  1618			return 1;
  1619	
  1620		return 0;
  1621	}
  1622	
  1623	int btrfs_qgroup_trace_subtree(struct btrfs_trans_handle *trans,
  1624				       struct btrfs_root *root,

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