Re: [PATCH 5/8] xfs: Protect xfs_file_aio_write() & xfs_setattr_size() with sb_start_write - sb_end_write

Dave Chinner
2012-01-24T07:19:31Z

On Fri, Jan 20, 2012 at 09:34:43PM +0100, Jan Kara wrote:
> Replace racy xfs_wait_for_freeze() check in xfs_file_aio_write() with
> a reliable sb_start_write() - sb_end_write() locking. Due to lock ranking
> dictated by the page fault code we have to call sb_start_write() after we
> acquire ilock.

It appears to me that you have indeed confused the ilock with the
iolock.

> Similarly we have to protect xfs_setattr_size() because it can modify last
> page of truncated file. Because ilock is dropped in xfs_setattr_size() we
> have to drop and retake write access as well to avoid deadlocks.

>
> CC: Ben Myers <bpm@sgi.com>
> CC: Alex Elder <elder@kernel.org>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/xfs/xfs_file.c | 6 ++++--
> fs/xfs/xfs_iops.c | 6 ++++++
> 2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 753ed9b..9efd153 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -862,9 +862,11 @@ xfs_file_dio_aio_write(
> *iolock = XFS_IOLOCK_SHARED;
> }
>
> + sb_start_write(inode->i_sb, SB_FREEZE_WRITE);
> trace_xfs_file_direct_write(ip, count, iocb->ki_pos, 0);
> ret = generic_file_direct_write(iocb, iovp,
> &nr_segs, pos, &iocb->ki_pos, count, ocount);
> + sb_end_write(inode->i_sb, SB_FREEZE_WRITE);

That's inside the iolock, not the ilock. Either way, it is
incorrect. This accounting should be outside the iolock - because
xfs_trans_alloc() can be called with the iolock held. Therefore the
freeze/lock order needs to be

sb_start_write(SB_FREEZE_WRITE)
XFS(ip)->i_iolock
XFS(ip)->i_ilock
sb_end_write(SB_FREEZE_WRITE)

Which matches the current freeze/lock order.

> @@ -945,8 +949,6 @@ xfs_file_aio_write(
> if (ocount == 0)
> return 0;
>
> - xfs_wait_for_freeze(ip->i_mount, SB_FREEZE_WRITE);
> -

that's where sb_start_write() needs to be, and the sb-end_write()
call needs to below the generic_write_sync() calls that will trigger
IO on O_SYNC writes. Otherwise it is not covering all the IO path
correctly.

> if (XFS_FORCED_SHUTDOWN(ip->i_mount))
> return -EIO;
>
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 3579bc8..798b9c6 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -793,6 +793,7 @@ xfs_setattr_size(
> return xfs_setattr_nonsize(ip, iattr, 0);
> }
>
> + sb_start_write(inode->i_sb, SB_FREEZE_WRITE);
> /*
> * Make sure that the dquots are attached to the inode.
> */
> @@ -849,10 +850,14 @@ xfs_setattr_size(
> xfs_get_blocks);
> if (error)
> goto out_unlock;
> + /* Drop the write access to avoid lock inversion with ilock */
> + sb_end_write(inode->i_sb, SB_FREEZE_WRITE);
>
> xfs_ilock(ip, XFS_ILOCK_EXCL);
> lock_flags |= XFS_ILOCK_EXCL;
>
> + sb_start_write(inode->i_sb, SB_FREEZE_WRITE);
> +

This is caused by the previous problems I pointed out. You should
not need to drop the freeze reference here at all.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com