--- //depot/vendor/freebsd/src/sys/kern/kern_rwlock.c	2007/11/26 22:44:43
+++ //depot/user/attilio/attilio_schedlock/kern/kern_rwlock.c	2007/11/29 20:29:57
@@ -238,19 +238,9 @@
 	KASSERT(rw_wowner(rw) != curthread,
 	    ("%s (%s): wlock already held @ %s:%d", __func__,
 	    rw->lock_object.lo_name, file, line));
-	WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line);
+	WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_NORECURSE,
+	    file, line);
 
-	/*
-	 * Note that we don't make any attempt to try to block read
-	 * locks once a writer has blocked on the lock.  The reason is
-	 * that we currently allow for read locks to recurse and we
-	 * don't keep track of all the holders of read locks.  Thus, if
-	 * we were to block readers once a writer blocked and a reader
-	 * tried to recurse on their reader lock after a writer had
-	 * blocked we would end up in a deadlock since the reader would
-	 * be blocked on the writer, and the writer would be blocked
-	 * waiting for the reader to release its original read lock.
-	 */
 	for (;;) {
 		/*
 		 * Handle the easy case.  If no other thread has a write
--- //depot/vendor/freebsd/src/sys/kern/subr_witness.c	2007/11/24 04:37:19
+++ //depot/user/attilio/attilio_schedlock/kern/subr_witness.c	2007/11/29 20:29:57
@@ -922,6 +922,9 @@
 			    lock1->li_file, lock1->li_line);
 			panic("excl->share");
 		}
+		if (flags & LOP_NORECURSE)
+			panic("recurse on non-recursive lock %s @ %s:%d",
+			    lock->lo_name, file, line);
 		return;
 	}
 
--- //depot/vendor/freebsd/src/sys/sys/lock.h	2007/11/18 14:44:31
+++ //depot/user/attilio/attilio_schedlock/sys/lock.h	2007/11/29 20:29:57
@@ -103,6 +103,7 @@
 #define	LOP_TRYLOCK	0x00000004	/* Don't check lock order. */
 #define	LOP_EXCLUSIVE	0x00000008	/* Exclusive lock. */
 #define	LOP_DUPOK	0x00000010	/* Don't check for duplicate acquires */
+#define	LOP_NORECURSE	0x00000020	/* Don't allow recursion for the lock */
 
 /* Flags passed to witness_assert. */
 #define	LA_UNLOCKED	0x00000000	/* Lock is unlocked. */