Re: [Bug 207713] New: xfs: data races on ip->i_itemp->ili_fields in xfs_inode_clean()

[Dave Chinner <david@fromorbit.com>]

On Wed, May 13, 2020 at 03:02:10AM +0000, bugzilla-daemon@bugzilla.kernel.org wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=207713
> 
>             Bug ID: 207713
>            Summary: xfs: data races on ip->i_itemp->ili_fields in
>                     xfs_inode_clean()
>            Product: File System
>            Version: 2.5
>     Kernel Version: 5.4
>           Hardware: All
>                 OS: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: XFS
>           Assignee: filesystem_xfs@kernel-bugs.kernel.org
>           Reporter: baijiaju1990@gmail.com
>         Regression: No
> 
> The function xfs_inode_clean() is concurrently executed with the functions
> xfs_inode_item_format_data_fork(), xfs_trans_log_inode() and
> xfs_inode_item_format() at runtime in the following call contexts:
> 
> Thread 1:
> xfsaild()
>   xfsaild_push()
>     xfsaild_push_item()
>       xfs_inode_item_push()
>         xfs_iflush()
>           xfs_iflush_cluster()
>             xfs_inode_clean()

The code explains:

                /*
                 * Do an un-protected check to see if the inode is dirty and
                 * is a candidate for flushing.  These checks will be repeated
                 * later after the appropriate locks are acquired.
                 */
                if (xfs_inode_clean(cip) && xfs_ipincount(cip) == 0)
                        continue;

Then it is repeated if the racy check indicates the inode is dirty
whilst holding the correct inode locks.  All three of the "thread 2"
cases are modifying the fields with the correct locks held, so there
isn't actually a data race bug we care about here.

IOWs, this code is simply avoiding taking locks if we don't need
them - it's a pattern we use in numerous places in XFS, so you're
going to get lots of false positives from your tool.

Not a bug, please close.

-Dave.
-- 
Dave Chinner
david@fromorbit.com