Improve VFS locking:

- Implement real draining for vfs consumers by not relying on the
  mnt_lock and using instead a refcount in order to keep track of lock
  requesters.
- Due to the change above, remove the mnt_lock lockmgr because it is now
  useless.
- Due to the change above, vfs_busy() is no more linked to a lockmgr.
  Change so its KPI by removing the interlock argument and defining 2 new
  flags for it: MBF_NOWAIT which basically replaces the LK_NOWAIT of the
  old version (which was unlinked from the lockmgr alredy) and
  MBF_MNTLSTLOCK which provides the ability to drop the mountlist_mtx
  once the mnt interlock is held (ability still desired by most consumers).
- The stub used into vfs_mount_destroy(), that allows to override the
  mnt_ref if running for more than 3 seconds, make it totally useless.
  Remove it as it was thought to work into older versions.
  If a problem of "refcount held never going away" should appear, we will
  need to fix properly instead than trust on such hackish solution.
- Fix a bug where returning (with an error) from dounmount() was still
  leaving the MNTK_MWAIT flag on even if it the waiters were actually
  woken up. Just a place in vfs_mount_destroy() is left because it is
  going to recycle the structure in any case, so it doesn't matter.
- Remove the markercnt refcount as it is useless.

This patch modifies VFS ABI and breaks KPI for vfs_busy() so manpages and
__FreeBSD_version will be modified accordingly.

Discussed with:	kib
Tested by:	pho

1 files changed, 4 insertions, 4 deletions

diff --git a/sys/kern/vfs_syscalls.c b/sys/kern/vfs_syscalls.c
index 4d40f35..32dc4d1 100644
--- a/sys/kern/vfs_syscalls.c
+++ b/sys/kern/vfs_syscalls.c
@@ -124,7 +124,7 @@ sync(td, uap)
 
 	mtx_lock(&mountlist_mtx);
 	for (mp = TAILQ_FIRST(&mountlist); mp != NULL; mp = nmp) {
-		if (vfs_busy(mp, LK_NOWAIT, &mountlist_mtx)) {
+		if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK)) {
 			nmp = TAILQ_NEXT(mp, mnt_list);
 			continue;
 		}
@@ -197,7 +197,7 @@ quotactl(td, uap)
 	vfslocked = NDHASGIANT(&nd);
 	NDFREE(&nd, NDF_ONLY_PNBUF);
 	mp = nd.ni_vp->v_mount;
-	if ((error = vfs_busy(mp, 0, NULL))) {
+	if ((error = vfs_busy(mp, 0))) {
 		vrele(nd.ni_vp);
 		VFS_UNLOCK_GIANT(vfslocked);
 		return (error);
@@ -479,7 +479,7 @@ kern_getfsstat(struct thread *td, struct statfs **buf, size_t bufsize,
 			continue;
 		}
 #endif
-		if (vfs_busy(mp, LK_NOWAIT, &mountlist_mtx)) {
+		if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK)) {
 			nmp = TAILQ_NEXT(mp, mnt_list);
 			continue;
 		}
@@ -741,7 +741,7 @@ fchdir(td, uap)
 	error = change_dir(vp, td);
 	while (!error && (mp = vp->v_mountedhere) != NULL) {
 		int tvfslocked;
-		if (vfs_busy(mp, 0, 0))
+		if (vfs_busy(mp, 0))
 			continue;
 		tvfslocked = VFS_LOCK_GIANT(mp);
 		error = VFS_ROOT(mp, LK_EXCLUSIVE, &tdp, td);