Revert previous attempt at serializing `uo_refs'.

It is incorrect to look at/modify `uo_refs' without lock in uvm_vnp_sync().
However sleeping is not possible and spinning introduces a deadlock with the
KERNEL_LOCK() when another thread faults on a mmaped region related to a
vnode we're currently synching.

A proper fix should be investigated.  For now a kown a race, mostly safe due
to the KERNEL_LOCK(), is better than a possible deadlock.

Issue reported by caspar@

Discussed with and ok semarie@

diff --git a/sys/uvm/uvm_vnode.c b/sys/uvm/uvm_vnode.c
index 8824d62..b35e864 100644 (file)
--- a/sys/uvm/uvm_vnode.c
+++ b/sys/uvm/uvm_vnode.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: uvm_vnode.c,v 1.124 2022/05/03 21:20:35 bluhm Exp $   */
+/*     $OpenBSD: uvm_vnode.c,v 1.125 2022/07/07 13:52:20 mpi Exp $     */
 /*     $NetBSD: uvm_vnode.c,v 1.36 2000/11/24 20:34:01 chs Exp $       */
 
 /*
@@ -1472,11 +1472,6 @@ uvm_vnp_sync(struct mount *mp)
                if (mp && vp->v_mount != mp)
                        continue;
 
-               /* Spin to ensure `uvn_wlist' isn't modified concurrently. */
-               while (rw_enter(uvn->u_obj.vmobjlock, RW_WRITE|RW_NOSLEEP)) {
-                       CPU_BUSY_CYCLE();
-               }
-
                /*
                 * If the vnode is "blocked" it means it must be dying, which
                 * in turn means its in the process of being flushed out so
@@ -1485,10 +1480,8 @@ uvm_vnp_sync(struct mount *mp)
                 * note that uvn must already be valid because we found it on
                 * the wlist (this also means it can't be ALOCK'd).
                 */
-               if ((uvn->u_flags & UVM_VNODE_BLOCKED) != 0) {
-                       rw_exit(uvn->u_obj.vmobjlock);
+               if ((uvn->u_flags & UVM_VNODE_BLOCKED) != 0)
                        continue;
-               }
 
                /*
                 * gain reference.   watch out for persisting uvns (need to
@@ -1497,7 +1490,6 @@ uvm_vnp_sync(struct mount *mp)
                if (uvn->u_obj.uo_refs == 0)
                        vref(vp);
                uvn->u_obj.uo_refs++;
-               rw_exit(uvn->u_obj.vmobjlock);
 
                SIMPLEQ_INSERT_HEAD(&uvn_sync_q, uvn, u_syncq);
        }