gfs2: fix race between gfs2_freeze_func and unmount

As part of the freeze operation, gfs2_freeze_func() is left blocking
on a request to hold the sd_freeze_gl in SH. This glock is held in EX
by the gfs2_freeze() code.

A subsequent call to gfs2_unfreeze() releases the EXclusively held
sd_freeze_gl, which allows gfs2_freeze_func() to acquire it in SH and
resume its operation.

gfs2_unfreeze(), however, doesn't wait for gfs2_freeze_func() to complete.
If a umount is issued right after unfreeze, it could result in an
inconsistent filesystem because some journal data (statfs update) isn't
written out.

Refer to commit 2497255 for a more detailed explanation of how
freeze/unfreeze work.

This patch causes gfs2_unfreeze() to wait for gfs2_freeze_func() to
complete before returning to the user.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/incore.h

@@ -621,6 +621,7 @@ enum {
 	SDF_SKIP_DLM_UNLOCK	= 8,
 	SDF_FORCE_AIL_FLUSH     = 9,
 	SDF_AIL1_IO_ERROR	= 10,
+	SDF_FS_FROZEN           = 11,
 };
 
 enum gfs2_freeze_state {

fs/gfs2/super.c

@@ -973,8 +973,7 @@ void gfs2_freeze_func(struct work_struct *work)
 	if (error) {
 		printk(KERN_INFO "GFS2: couldn't get freeze lock : %d\n", error);
 		gfs2_assert_withdraw(sdp, 0);
-	}
-	else {
+	} else {
 		atomic_set(&sdp->sd_freeze_state, SFS_UNFROZEN);
 		error = thaw_super(sb);
 		if (error) {
@@ -987,6 +986,8 @@ void gfs2_freeze_func(struct work_struct *work)
 		gfs2_glock_dq_uninit(&freeze_gh);
 	}
 	deactivate_super(sb);
+	clear_bit_unlock(SDF_FS_FROZEN, &sdp->sd_flags);
+	wake_up_bit(&sdp->sd_flags, SDF_FS_FROZEN);
 	return;
 }
 
@@ -1029,6 +1030,7 @@ static int gfs2_freeze(struct super_block *sb)
 		msleep(1000);
 	}
 	error = 0;
+	set_bit(SDF_FS_FROZEN, &sdp->sd_flags);
 out:
 	mutex_unlock(&sdp->sd_freeze_mutex);
 	return error;
@@ -1053,7 +1055,7 @@ static int gfs2_unfreeze(struct super_block *sb)
 
 	gfs2_glock_dq_uninit(&sdp->sd_freeze_gh);
 	mutex_unlock(&sdp->sd_freeze_mutex);
-	return 0;
+	return wait_on_bit(&sdp->sd_flags, SDF_FS_FROZEN, TASK_INTERRUPTIBLE);
 }
 
 /**