diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -658,6 +658,7 @@
 			error = EBADF;
 			break;
 		}
+		fsetfl_lock(fp);
 		do {
 			tmp = flg = fp->f_flag;
 			tmp &= ~FCNTLFLAGS;
@@ -677,6 +678,7 @@
 			if (error != 0)
 				goto revert_nonblock;
 		}
+		fsetfl_unlock(fp);
 		fdrop(fp, td);
 		break;
 revert_nonblock:
@@ -691,6 +693,7 @@
 			tmp |= got_cleared;
 			tmp &= ~got_set;
 		} while (atomic_cmpset_int(&fp->f_flag, flg, tmp) == 0);
+		fsetfl_unlock(fp);
 		fdrop(fp, td);
 		break;
 
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
@@ -896,6 +896,18 @@
 	return (atomic_load_long(&fp->f_offset));
 }
 
+void
+fsetfl_lock(struct file *fp)
+{
+	file_v_lock(fp, FILE_V_SETFL_LOCKED, FILE_V_SETFL_LOCK_WAITING);
+}
+
+void
+fsetfl_unlock(struct file *fp)
+{
+	file_v_unlock(fp, FILE_V_SETFL_LOCKED, FILE_V_SETFL_LOCK_WAITING);
+}
+
 #else	/* OFF_MAX <= LONG_MAX */
 
 static void
@@ -971,6 +983,30 @@
 
 	return (foffset_lock(fp, FOF_NOLOCK));
 }
+
+void
+fsetfl_lock(struct file *fp)
+{
+	struct mtx *mtxp;
+
+	mtxp = mtx_pool_find(mtxpool_sleep, fp);
+	mtx_lock(mtxp);
+	file_v_lock_mtxp(fp, mtxp, FILE_V_SETFL_LOCKED,
+	    FILE_V_SETFL_LOCK_WAITING);
+	mtx_unlock(mtxp);
+}
+
+void
+fsetfl_unlock(struct file *fp)
+{
+	struct mtx *mtxp;
+
+	mtxp = mtx_pool_find(mtxpool_sleep, fp);
+	mtx_lock(mtxp);
+	file_v_unlock_mtxp(fp, mtxp, FILE_V_SETFL_LOCKED,
+	    FILE_V_SETFL_LOCK_WAITING);
+	mtx_unlock(mtxp);
+}
 #endif
 
 void
diff --git a/sys/sys/file.h b/sys/sys/file.h
--- a/sys/sys/file.h
+++ b/sys/sys/file.h
@@ -93,6 +93,8 @@
 void foffset_lock_uio(struct file *fp, struct uio *uio, int flags);
 void foffset_unlock(struct file *fp, off_t val, int flags);
 void foffset_unlock_uio(struct file *fp, struct uio *uio, int flags);
+void fsetfl_lock(struct file *fp);
+void fsetfl_unlock(struct file *fp);
 
 static inline off_t
 foffset_get(struct file *fp)
@@ -222,6 +224,8 @@
 
 #define	FILE_V_FOFFSET_LOCKED		0x0001
 #define	FILE_V_FOFFSET_LOCK_WAITING	0x0002
+#define	FILE_V_SETFL_LOCKED		0x0004
+#define	FILE_V_SETFL_LOCK_WAITING	0x0008
 #endif /* __BSD_VISIBLE */
 
 #endif /* _KERNEL || _WANT_FILE */