diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -770,6 +770,8 @@
 	flagsp = &fp->f_vnread_flags;
 	if (atomic_cmpset_acq_16(flagsp, 0, FOFFSET_LOCKED))
 		return (atomic_load_long(&fp->f_offset));
+	if ((flags & FOF_NONBLOCK) != 0)
+		return (-1);
 
 	sleepq_lock(&fp->f_vnread_flags);
 	state = atomic_load_16(flagsp);
@@ -804,6 +806,7 @@
 	short state;
 
 	KASSERT((flags & FOF_OFFSET) == 0, ("FOF_OFFSET passed"));
+	KASSERT(val >= 0, ("negative offset"));
 
 	if ((flags & FOF_NOUPDATE) == 0)
 		atomic_store_long(&fp->f_offset, val);
@@ -848,6 +851,10 @@
 	mtx_lock(mtxp);
 	if ((flags & FOF_NOLOCK) == 0) {
 		while (fp->f_vnread_flags & FOFFSET_LOCKED) {
+			if ((flags & FOF_NONBLOCK) != 0) {
+				mtx_unlock(mtxp);
+				return (-1);
+			}
 			fp->f_vnread_flags |= FOFFSET_LOCK_WAITING;
 			msleep(&fp->f_vnread_flags, mtxp, PUSER -1,
 			    "vofflock", 0);
diff --git a/sys/sys/file.h b/sys/sys/file.h
--- a/sys/sys/file.h
+++ b/sys/sys/file.h
@@ -85,6 +85,7 @@
 #define	FOF_NEXTOFF_R	0x04	/* Also update f_nextoff[UIO_READ] */
 #define	FOF_NEXTOFF_W	0x08	/* Also update f_nextoff[UIO_WRITE] */
 #define	FOF_NOUPDATE	0x10	/* Do not update f_offset */
+#define	FOF_NONBLOCK	0x20	/* Do not block if the offset is locked */
 off_t foffset_lock(struct file *fp, int flags);
 void foffset_lock_uio(struct file *fp, struct uio *uio, int flags);
 void foffset_unlock(struct file *fp, off_t val, int flags);