new vfs freeze hooks and gfs2 freeze rewrite

The existing gfs2 freeze code can't properly make use the kernel vfs freezing code on all the nodes of a cluster. This is because the only hook into the gfs2 freeze code comes inside of freeze_super, which holds the sb->s_umount lock. In order to notify the other nodes in the cluster to freeze the filesystem, gfs2 must grab a glock. Thus, if two nodes tried to freeze the filesystem at the same time and all cluster nodes used freeze_super to enforce the freeze, both nodes would first grab the sb->s_umount lock in freeze_super, and then they would race to grab the gfs2 freeze glock. The loser would be stuck in freeze_super, waiting on the freeze glock. This means that gfs2 can't rely on freezing all the nodes in the cluster by calling freeze_super, because one node would already be stuck in that function with sb->s_umount held. So, currently gfs2 only calls freeze_super on the node that initiates the freeze and deals with blocking the writes entirely within the gfs2 code. This involves some pretty hacky code.

To fix this, the first patch of this patchset adds two new vfs hooks, freeze_super and thaw_super. If a filesystem implements these hooks, they will be called instead of the vfs freeze_super and thaw_super functions. The second patch of this set makes use of these hooks to simplify the gfs2 freezing code. This allows gfs2 to grab the freeze glock first and call the actual vfs freeze functions in response to node grabbing the freeze glock. Thus, all nodes can use the vfs freeze_super and thaw_super code to freeze the filesystem. This vesion also fixes some issues with a race between reserving log space and making the filesystem read-only.

The current hook names were suggested by Jan Kara, over my original prepare_freeeze and prepare_thaw names, which were more confusing.

Benjamin Marzinski (2):
fs: add freeze_super/thaw_super fs hooks
gfs2: update freeze code to use freeze/thaw_super on all nodes

  fs/block_dev.c       |  10 ++++-
  fs/gfs2/glops.c      |  26 ++++++------
  fs/gfs2/glops.h      |   2 +
  fs/gfs2/incore.h     |  18 ++++++---
  fs/gfs2/inode.c      |  40 +++++-------------
  fs/gfs2/log.c        |  42 +++++++++----------
  fs/gfs2/main.c       |  11 ++++-
  fs/gfs2/ops_fstype.c |  18 +++------
  fs/gfs2/super.c      | 112 ++++++++++++++++++++++++++++++++++-----------------
  fs/gfs2/super.h      |   1 +
  fs/gfs2/trans.c      |  17 ++++++--
  fs/ioctl.c           |   6 ++-
  include/linux/fs.h   |   2 +
  13 files changed, 176 insertions(+), 129 deletions(-)