[src/trunk]: src/sys/kern pserialize(9): Lift rule that pserialize_perform be...

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

details:   https://anonhg.NetBSD.org/src/rev/1bb6ffc2de46
branches:  trunk
changeset: 989033:1bb6ffc2de46
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Oct 10 11:20:46 2021 +0000

description:
pserialize(9): Lift rule that pserialize_perform be serialized.

There may have been a technical reason for this back when we were
following the expired passive serialization patent to the letter, but
no more -- and this is a real burden for some applications.

diffstat:

 share/man/man9/pserialize.9 |  14 ++++++++------
 sys/kern/subr_pserialize.c  |  16 ++++------------
 2 files changed, 12 insertions(+), 18 deletions(-)

diffs (109 lines):

diff -r a8dcee6ed41e -r 1bb6ffc2de46 share/man/man9/pserialize.9
--- a/share/man/man9/pserialize.9       Sun Oct 10 11:20:29 2021 +0000
+++ b/share/man/man9/pserialize.9       Sun Oct 10 11:20:46 2021 +0000
@@ -1,4 +1,4 @@
-.\"    $NetBSD: pserialize.9,v 1.13 2017/07/03 21:28:48 wiz Exp $
+.\"    $NetBSD: pserialize.9,v 1.14 2021/10/10 11:20:46 riastradh Exp $
 .\"
 .\" Copyright (c) 2011 The NetBSD Foundation, Inc.
 .\" All rights reserved.
@@ -67,9 +67,10 @@
 .It Fn pserialize_perform
 Perform the passive serialization on the writer side.
 Passing of this function ensures that no readers are in action.
-Writers must be additionally serialized with a separate mechanism,
-e.g.
-.Xr mutex 9 .
+Writers are typically additionally serialized with a separate
+mechanism, e.g.
+.Xr mutex 9 ,
+to remove objects used by readers from a published list.
 Operation blocks and it may only be performed from thread context.
 .El
 .\" -----
@@ -152,14 +153,15 @@
                        break;
                }
        }
+       mutex_exit(&frobbotzim.lock);
+
        /*
         * Wait for all existing readers to complete.  New readers will
         * not see f because the list no longer points to it.
         */
        pserialize_perform(frobbotzim.psz);
+
        /* Now nobody else can be touching f, so it is safe to free.  */
-       mutex_exit(&frobbotzim.lock);
-
        if (f != NULL)
                pool_put(&frotz_pool, f);
 .Ed
diff -r a8dcee6ed41e -r 1bb6ffc2de46 sys/kern/subr_pserialize.c
--- a/sys/kern/subr_pserialize.c        Sun Oct 10 11:20:29 2021 +0000
+++ b/sys/kern/subr_pserialize.c        Sun Oct 10 11:20:46 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: subr_pserialize.c,v 1.17 2019/12/05 03:21:29 riastradh Exp $   */
+/*     $NetBSD: subr_pserialize.c,v 1.18 2021/10/10 11:20:46 riastradh Exp $   */
 
 /*-
  * Copyright (c) 2010, 2011 The NetBSD Foundation, Inc.
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_pserialize.c,v 1.17 2019/12/05 03:21:29 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_pserialize.c,v 1.18 2021/10/10 11:20:46 riastradh Exp $");
 
 #include <sys/param.h>
 #include <sys/atomic.h>
@@ -43,7 +43,7 @@
 #include <sys/xcall.h>
 
 struct pserialize {
-       lwp_t *                 psz_owner;
+       char                    psz_dummy;
 };
 
 static kmutex_t                        psz_lock        __cacheline_aligned;
@@ -86,16 +86,13 @@
 pserialize_destroy(pserialize_t psz)
 {
 
-       KASSERT(psz->psz_owner == NULL);
        kmem_free(psz, sizeof(*psz));
 }
 
 /*
  * pserialize_perform:
  *
- *     Perform the write side of passive serialization.  This operation
- *     MUST be serialized at a caller level (e.g. with a mutex or by a
- *     single-threaded use).
+ *     Perform the write side of passive serialization.
  */
 void
 pserialize_perform(pserialize_t psz)
@@ -107,22 +104,17 @@
        if (__predict_false(panicstr != NULL)) {
                return;
        }
-       KASSERT(psz->psz_owner == NULL);
 
        if (__predict_false(mp_online == false)) {
                psz_ev_excl.ev_count++;
                return;
        }
 
-       psz->psz_owner = curlwp;
-
        /*
         * Broadcast a NOP to all CPUs and wait until all of them complete.
         */
        xc_barrier(XC_HIGHPRI);
 
-       KASSERT(psz->psz_owner == curlwp);
-       psz->psz_owner = NULL;
        mutex_enter(&psz_lock);
        psz_ev_excl.ev_count++;
        mutex_exit(&psz_lock);