Decrement uobj reference count without KERNEL_LOCK().

Reduce KERNEL_LOCK() contention when tearing down file-backed regions.  Here
it is safe to interleave the KERNEL_LOCK() and a rwlock because the former
is released if the latter is contented.

Contention analysed by and ok claudio@, ok kettenis@

diff --git a/sys/uvm/uvm_vnode.c b/sys/uvm/uvm_vnode.c
index 079186c..8213f26 100644 (file)
--- a/sys/uvm/uvm_vnode.c
+++ b/sys/uvm/uvm_vnode.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: uvm_vnode.c,v 1.133 2024/07/24 12:16:21 mpi Exp $     */
+/*     $OpenBSD: uvm_vnode.c,v 1.134 2024/10/23 07:18:44 mpi Exp $     */
 /*     $NetBSD: uvm_vnode.c,v 1.36 2000/11/24 20:34:01 chs Exp $       */
 
 /*
@@ -306,15 +306,14 @@ uvn_detach(struct uvm_object *uobj)
        struct vnode *vp;
        int oldflags;
 
-       KERNEL_LOCK();
        rw_enter(uobj->vmobjlock, RW_WRITE);
        uobj->uo_refs--;                        /* drop ref! */
        if (uobj->uo_refs) {                    /* still more refs */
                rw_exit(uobj->vmobjlock);
-               KERNEL_UNLOCK();
                return;
        }
 
+       KERNEL_LOCK();
        /* get other pointers ... */
        uvn = (struct uvm_vnode *) uobj;
        vp = uvn->u_vnode;