GFS2: remove transaction glock

GFS2 has a transaction glock, which must be grabbed for every
transaction, whose purpose is to deal with freezing the filesystem.
Aside from this involving a large amount of locking, it is very easy to
make the current fsfreeze code hang on unfreezing.

This patch rewrites how gfs2 handles freezing the filesystem. The
transaction glock is removed. In it's place is a freeze glock, which is
cached (but not held) in a shared state by every node in the cluster
when the filesystem is mounted. This lock only needs to be grabbed on
freezing, and actions which need to be safe from freezing, like
recovery.

When a node wants to freeze the filesystem, it grabs this glock
exclusively.  When the freeze glock state changes on the nodes (either
from shared to unlocked, or shared to exclusive), the filesystem does a
special log flush.  gfs2_log_flush() does all the work for flushing out
the and shutting down the incore log, and then it tries to grab the
freeze glock in a shared state again.  Since the filesystem is stuck in
gfs2_log_flush, no new transaction can start, and nothing can be written
to disk. Unfreezing the filesytem simply involes dropping the freeze
glock, allowing gfs2_log_flush() to grab and then release the shared
lock, so it is cached for next time.

However, in order for the unfreezing ioctl to occur, gfs2 needs to get a
shared lock on the filesystem root directory inode to check permissions.
If that glock has already been grabbed exclusively, fsfreeze will be
unable to get the shared lock and unfreeze the filesystem.

In order to allow the unfreeze, this patch makes gfs2 grab a shared lock
on the filesystem root directory during the freeze, and hold it until it
unfreezes the filesystem.  The functions which need to grab a shared
lock in order to allow the unfreeze ioctl to be issued now use the lock
grabbed by the freeze code instead.

The freeze and unfreeze code take care to make sure that this shared
lock will not be dropped while another process is using it.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index bdf70c18610c..2434a96f95df 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -465,9 +465,7 @@ struct gfs2_trans {
 	unsigned int tr_reserved;
 	unsigned int tr_touched:1;
 	unsigned int tr_attached:1;
-
-	struct gfs2_holder tr_t_gh;
-
+	unsigned int tr_alloced:1;
 
 	unsigned int tr_num_buf_new;
 	unsigned int tr_num_databuf_new;
@@ -682,7 +680,7 @@ struct gfs2_sbd {
 	struct lm_lockstruct sd_lockstruct;
 	struct gfs2_holder sd_live_gh;
 	struct gfs2_glock *sd_rename_gl;
-	struct gfs2_glock *sd_trans_gl;
+	struct gfs2_glock *sd_freeze_gl;
 	wait_queue_head_t sd_glock_wait;
 	atomic_t sd_glock_disposal;
 	struct completion sd_locking_init;
@@ -794,6 +792,12 @@ struct gfs2_sbd {
 
 	/* For quiescing the filesystem */
 	struct gfs2_holder sd_freeze_gh;
+	struct gfs2_holder sd_freeze_root_gh;
+	struct gfs2_holder sd_thaw_gh;
+	atomic_t sd_log_freeze;
+	atomic_t sd_frozen_root;
+	wait_queue_head_t sd_frozen_root_wait;
+	wait_queue_head_t sd_log_frozen_wait;
 
 	char sd_fsname[GFS2_FSNAME_LEN];
 	char sd_table_name[GFS2_FSNAME_LEN];