Move parts of the commit log for r166167, where Tor explained the

interaction between vnode locks and vfs_busy(), into comment.

MFC after:	1 week

1 files changed, 32 insertions, 0 deletions

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 325ca99..85d176f 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -348,6 +348,38 @@ SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_FIRST, vntblinit, NULL);
 /*
  * Mark a mount point as busy. Used to synchronize access and to delay
  * unmounting. Eventually, mountlist_mtx is not released on failure.
+ *
+ * vfs_busy() is a custom lock, it can block the caller.
+ * vfs_busy() only sleeps if the unmount is active on the mount point.
+ * For a mountpoint mp, vfs_busy-enforced lock is before lock of any
+ * vnode belonging to mp.
+ *
+ * Lookup uses vfs_busy() to traverse mount points.
+ * root fs			var fs
+ * / vnode lock		A	/ vnode lock (/var)		D
+ * /var vnode lock	B	/log vnode lock(/var/log)	E
+ * vfs_busy lock	C	vfs_busy lock			F
+ *
+ * Within each file system, the lock order is C->A->B and F->D->E.
+ *
+ * When traversing across mounts, the system follows that lock order:
+ *
+ *        C->A->B
+ *              |
+ *              +->F->D->E
+ *
+ * The lookup() process for namei("/var") illustrates the process:
+ *  VOP_LOOKUP() obtains B while A is held
+ *  vfs_busy() obtains a shared lock on F while A and B are held
+ *  vput() releases lock on B
+ *  vput() releases lock on A
+ *  VFS_ROOT() obtains lock on D while shared lock on F is held
+ *  vfs_unbusy() releases shared lock on F
+ *  vn_lock() obtains lock on deadfs vnode vp_crossmp instead of A.
+ *    Attempt to lock A (instead of vp_crossmp) while D is held would
+ *    violate the global order, causing deadlocks.
+ *
+ * dounmount() locks B while F is drained.
  */
 int
 vfs_busy(struct mount *mp, int flags)