[src/trunk]: src/sys/arch/xen/xen As noted by rmind@, use the _nv() to fetch ...

[jym <jym%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/6340a2076e6a
branches:  trunk
changeset: 765004:6340a2076e6a
user:      jym <jym%NetBSD.org@localhost>
date:      Sun May 15 20:58:54 2011 +0000

description:
As noted by rmind@, use the _nv() to fetch the new value. A race is
possible between the decrement and the fetch of the ref counter value,
hence we might call the G/C routine twice. Not good.

Also remove the 'volatile' attribute, refcnt is only use by xbdi_put/_get
and should not be exposed anywhere else (except for initialization).

diffstat:

 sys/arch/xen/xen/xbdback_xenbus.c |  9 ++++-----
 1 files changed, 4 insertions(+), 5 deletions(-)

diffs (37 lines):

diff -r 90aae3706979 -r 6340a2076e6a sys/arch/xen/xen/xbdback_xenbus.c
--- a/sys/arch/xen/xen/xbdback_xenbus.c Sun May 15 18:33:33 2011 +0000
+++ b/sys/arch/xen/xen/xbdback_xenbus.c Sun May 15 20:58:54 2011 +0000
@@ -1,4 +1,4 @@
-/*      $NetBSD: xbdback_xenbus.c,v 1.35 2011/05/15 07:24:15 jym Exp $      */
+/*      $NetBSD: xbdback_xenbus.c,v 1.36 2011/05/15 20:58:54 jym Exp $      */
 
 /*
  * Copyright (c) 2006 Manuel Bouyer.
@@ -26,7 +26,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xbdback_xenbus.c,v 1.35 2011/05/15 07:24:15 jym Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xbdback_xenbus.c,v 1.36 2011/05/15 20:58:54 jym Exp $");
 
 #include <sys/types.h>
 #include <sys/param.h>
@@ -144,7 +144,7 @@
        grant_handle_t xbdi_ring_handle; /* to unmap the ring */
        vaddr_t xbdi_ring_va; /* to unmap the ring */
        /* disconnection must be postponed until all I/O is done */
-       volatile unsigned int xbdi_refcnt;
+       int xbdi_refcnt;
        /* 
         * State for I/O processing/coalescing follows; this has to
         * live here instead of on the stack because of the
@@ -171,8 +171,7 @@
 #define xbdi_get(xbdip) atomic_inc_uint(&(xbdip)->xbdi_refcnt)
 #define xbdi_put(xbdip)                                      \
 do {                                                         \
-       atomic_dec_uint(&(xbdip)->xbdi_refcnt);              \
-       if (0 == (xbdip)->xbdi_refcnt)                       \
+       if (atomic_dec_uint_nv(&(xbdip)->xbdi_refcnt) == 0)  \
                xbdback_finish_disconnect(xbdip);             \
 } while (/* CONSTCOND */ 0)