Re: quotacheck deadlock?

[Brian Foster <bfoster@redhat.com>]

On Wed, Jul 19, 2017 at 11:58:04PM -0700, Darrick J. Wong wrote:
> Hi,
> 
> I ran the following sequence of commands on 4.13-rc1:
> 
> # mkfs.xfs -f /dev/sdf
> # xfs_db -x -c 'sb 0' -c 'addr rootino' -c 'write -d core.uid 4294967295' /dev/sdf
> # mount /dev/sdf -o usrquota
> 
> The kernel reports that it's starting quotacheck, but never finishes.
> echo t > /proc/sysrq produces this for the hung mount command:
> 
> mount           R  running task        0   988    895 0x00000000
> Call Trace:
>  ? sched_clock_cpu+0xa8/0xe0
>  ? xfs_qm_flush_one+0x3c/0x120 [xfs]
>  ? lock_acquire+0xac/0x200
>  ? lock_acquire+0xac/0x200
>  ? xfs_qm_flush_one+0x3c/0x120 [xfs]
>  ? xfs_qm_dquot_walk+0xa1/0x170 [xfs]
>  ? get_lock_stats+0x19/0x60
>  ? get_lock_stats+0x19/0x60
>  ? xfs_qm_dquot_walk+0xa1/0x170 [xfs]
>  ? xfs_qm_dquot_walk+0x125/0x170 [xfs]
>  ? radix_tree_gang_lookup+0xd1/0xf0
>  ? xfs_qm_shrink_count+0x20/0x20 [xfs]
>  ? xfs_qm_dquot_walk+0xbb/0x170 [xfs]
>  ? kfree+0x23f/0x2d0
>  ? kvfree+0x2a/0x40
>  ? xfs_bulkstat+0x315/0x680 [xfs]
>  ? xfs_qm_get_rtblks+0xa0/0xa0 [xfs]
>  ? xfs_qm_quotacheck+0x2bd/0x360 [xfs]
>  ? xfs_qm_mount_quotas+0x106/0x1f0 [xfs]
>  ? xfs_mountfs+0x6f2/0xb00 [xfs]
>  ? xfs_fs_fill_super+0x483/0x610 [xfs]
>  ? mount_bdev+0x180/0x1b0
>  ? xfs_finish_flags+0x150/0x150 [xfs]
>  ? xfs_fs_mount+0x15/0x20 [xfs]
>  ? mount_fs+0x14/0x80
>  ? vfs_kern_mount+0x67/0x170
>  ? do_mount+0x195/0xd00
>  ? kmem_cache_alloc_trace+0x231/0x2a0
>  ? SyS_mount+0x95/0xe0
>  ? entry_SYSCALL_64_fastpath+0x1f/0xbe
> 
> Any thoughts?  I'm not sure what's going on for sure, other than the
> call stack looks funny and it's midnight so I'm going to sleep. :)
> 

It looks like a problem with the loop in xfs_qm_dquot_walk(). The next
lookup index is calculated as:

	 next_index = be32_to_cpu(dqp->q_core.d_id) + 1;

... each time through the loop. With the uid written above, the +1
overflows the 32-bit next_index back to zero and the lookup starts over.
I suppose a simple fix might be to do something like the following.
Thoughts?

--- 8< ---

diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
index 6ce948c..f013c893 100644
--- a/fs/xfs/xfs_qm.c
+++ b/fs/xfs/xfs_qm.c
@@ -111,6 +111,8 @@ xfs_qm_dquot_walk(
 			skipped = 0;
 			break;
 		}
+		if (!next_index)
+			break;
 	}
 
 	if (skipped) {