Re: [PATCH 1/5] xfs: take the ilock around xfs_bmapi_read in xfs_zero_remaining_bytes

Ben Myers
2013-12-05T20:37:40Z

On Fri, Dec 06, 2013 at 07:31:15AM +1100, Dave Chinner wrote:
> On Thu, Dec 05, 2013 at 07:58:31AM -0800, Christoph Hellwig wrote:
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> >
> > Index: xfs/fs/xfs/xfs_bmap_util.c
> > ===================================================================
> > --- xfs.orig/fs/xfs/xfs_bmap_util.c 2013-12-05 11:37:57.791685284 +0100
> > +++ xfs/fs/xfs/xfs_bmap_util.c 2013-12-05 11:39:43.599683113 +0100
> > @@ -1147,6 +1147,7 @@ xfs_zero_remaining_bytes(
> > xfs_mount_t *mp = ip->i_mount;
> > int nimap;
> > int error = 0;
> > + uint lock_mode;
> >
> > /*
> > * Avoid doing I/O beyond eof - it's not necessary
> > @@ -1159,11 +1160,15 @@ xfs_zero_remaining_bytes(
> > if (endoff > XFS_ISIZE(ip))
> > endoff = XFS_ISIZE(ip);
> >
> > + lock_mode = xfs_ilock_map_shared(ip);
> > +
> > bp = xfs_buf_get_uncached(XFS_IS_REALTIME_INODE(ip) ?
> > mp->m_rtdev_targp : mp->m_ddev_targp,
> > BTOBB(mp->m_sb.sb_blocksize), 0);
>
> This now holds the ilock over data IO, which is not allowed to be
> done as data IO completion can require the ilock to be taken. Yes,
> the code specifically avoids all these problems, but the general
> rule is that ilock is only held over metadata IO operations, not
> data IO. If we need data IO serialisation, then we use the iolock.
>
> So, while this protects the extent tree, it also violates other
> long-standing conventions for locking. Given that the code is
> special, I'mnot opposed to making a special rule for this one
> function, but it needs to be commented as to why this is a valid
> thing to do in this function....

Maybe it would be better if the ilock could be taken and dropped within the
loop.