If the buffer lock has waiters after the buffer has changed identity then

getnewbuf() needs to drop the buffer in order to wake waiters that might
sleep on the buffer in the context of the old identity.

sys/kern/kern_lock.c

@@ -565,6 +565,21 @@ lockcount(lkp)
 	return (count);
 }
 
+/*
+ * Determine the number of waiters on a lock.
+ */
+int
+lockwaiters(lkp)
+	struct lock *lkp;
+{
+	int count;
+
+	mtx_lock(lkp->lk_interlock);
+	count = lkp->lk_waitcount;
+	mtx_unlock(lkp->lk_interlock);
+	return (count);
+}
+
 /*
  * Print out information about state of a lock. Used by VOP_PRINT
  * routines to display status about contained locks.

sys/kern/vfs_bio.c

@@ -1889,6 +1889,17 @@ getnewbuf(int slpflag, int slptimeo, int size, int maxsize)
 			goto restart;
 		}
 
+		/*
+		 * Notify any waiters for the buffer lock about
+		 * identity change by freeing the buffer.
+		 */
+		if (qindex == QUEUE_CLEAN && BUF_LOCKWAITERS(bp) > 0) {
+			bp->b_flags |= B_INVAL;
+			bfreekva(bp);
+			brelse(bp);
+			goto restart;
+		}
+
 		/*
 		 * If we are overcomitted then recover the buffer and its
 		 * KVM space.  This occurs in rare situations when multiple

sys/sys/buf.h

@@ -371,6 +371,17 @@ BUF_REFCNT(struct buf *bp)
 	return ret;
 }
 
+
+/*
+ * Find out the number of waiters on a lock.
+ */
+static __inline int BUF_LOCKWAITERS(struct buf *);
+static __inline int
+BUF_LOCKWAITERS(struct buf *bp)
+{
+	return (lockwaiters(&bp->b_lock));
+}
+
 #endif /* _KERNEL */
 
 struct buf_queue_head {

sys/sys/lockmgr.h

@@ -203,6 +203,7 @@ void	transferlockers(struct lock *, struct lock *);
 void	lockmgr_printinfo(struct lock *);
 int	lockstatus(struct lock *, struct thread *);
 int	lockcount(struct lock *);
+int	lockwaiters(struct lock *);
 #ifdef DDB
 int	lockmgr_chain(struct thread *td, struct thread **ownerp);
 #endif