Add snapshots to the fast filesystem. Most of the changes support

the gating of system calls that cause modifications to the underlying
filesystem. The gating can be enabled by any filesystem that needs
to consistently suspend operations by adding the vop_stdgetwritemount
to their set of vnops. Once gating is enabled, the function
vfs_write_suspend stops all new write operations to a filesystem,
allows any filesystem modifying system calls already in progress
to complete, then sync's the filesystem to disk and returns. The
function vfs_write_resume allows the suspended write operations to
begin again. Gating is not added by default for all filesystems as
for SMP systems it adds two extra locks to such critical kernel
paths as the write system call. Thus, gating should only be added
as needed.

Details on the use and current status of snapshots in FFS can be
found in /sys/ufs/ffs/README.snapshot so for brevity and timelyness
is not included here. Unless and until you create a snapshot file,
these changes should have no effect on your system (famous last words).

1 files changed, 12 insertions, 2 deletions

diff --git a/sys/ufs/ffs/ffs_vnops.c b/sys/ufs/ffs/ffs_vnops.c
index 539f302..eb6d621 100644
--- a/sys/ufs/ffs/ffs_vnops.c
+++ b/sys/ufs/ffs/ffs_vnops.c
@@ -95,6 +95,7 @@ vop_t **ffs_specop_p;
 static struct vnodeopv_entry_desc ffs_specop_entries[] = {
 	{ &vop_default_desc,		(vop_t *) ufs_vnoperatespec },
 	{ &vop_fsync_desc,		(vop_t *) ffs_fsync },
+	{ &vop_copyonwrite_desc,	(vop_t *) ffs_copyonwrite },
 	{ NULL, NULL }
 };
 static struct vnodeopv_desc ffs_specop_opv_desc =
@@ -129,11 +130,20 @@ ffs_fsync(ap)
 	} */ *ap;
 {
 	struct vnode *vp = ap->a_vp;
+	struct inode *ip = VTOI(vp);
 	struct buf *bp;
 	struct buf *nbp;
 	int s, error, wait, passes, skipmeta;
 	daddr_t lbn;
 
+	/*
+	 * Snapshots have to be unlocked so they do not deadlock
+	 * checking whether they need to copy their written buffers.
+	 * We always hold a reference, so they cannot be removed
+	 * out from underneath us.
+	 */
+	if (ip->i_flags & SF_SNAPSHOT)
+		VOP_UNLOCK(vp, 0, ap->a_p);
 	wait = (ap->a_waitfor == MNT_WAIT);
 	if (vn_isdisk(vp, NULL)) {
 		lbn = INT_MAX;
@@ -141,8 +151,6 @@ ffs_fsync(ap)
 		    (vp->v_specmountpoint->mnt_flag & MNT_SOFTDEP))
 			softdep_fsync_mountdev(vp);
 	} else {
-		struct inode *ip;
-		ip = VTOI(vp);
 		lbn = lblkno(ip->i_fs, (ip->i_size + ip->i_fs->fs_bsize - 1));
 	}
 
@@ -279,5 +287,7 @@ loop:
 	}
 	splx(s);
 	error = UFS_UPDATE(vp, wait);
+	if (ip->i_flags & SF_SNAPSHOT)
+		vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, ap->a_p);
 	return (error);
 }