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[src/trunk]: src/sys/kern Remove some unneeded memory barriers and reads of t...
details: https://anonhg.NetBSD.org/src/rev/02031a8e31b5
branches: trunk
changeset: 461422:02031a8e31b5
user: ad <ad%NetBSD.org@localhost>
date: Mon Nov 25 20:16:22 2019 +0000
description:
Remove some unneeded memory barriers and reads of the lock word.
Prodded by Mateusz Guzik.
diffstat:
sys/kern/kern_rwlock.c | 84 +++++++++++++++++++++++++++----------------------
1 files changed, 47 insertions(+), 37 deletions(-)
diffs (180 lines):
diff -r e40f1e8daec5 -r 02031a8e31b5 sys/kern/kern_rwlock.c
--- a/sys/kern/kern_rwlock.c Mon Nov 25 17:24:59 2019 +0000
+++ b/sys/kern/kern_rwlock.c Mon Nov 25 20:16:22 2019 +0000
@@ -1,7 +1,7 @@
-/* $NetBSD: kern_rwlock.c,v 1.54 2019/05/09 05:00:31 ozaki-r Exp $ */
+/* $NetBSD: kern_rwlock.c,v 1.55 2019/11/25 20:16:22 ad Exp $ */
/*-
- * Copyright (c) 2002, 2006, 2007, 2008, 2009 The NetBSD Foundation, Inc.
+ * Copyright (c) 2002, 2006, 2007, 2008, 2009, 2019 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This code is derived from software contributed to The NetBSD Foundation
@@ -38,7 +38,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_rwlock.c,v 1.54 2019/05/09 05:00:31 ozaki-r Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_rwlock.c,v 1.55 2019/11/25 20:16:22 ad Exp $");
#define __RWLOCK_PRIVATE
@@ -112,6 +112,19 @@
#define RW_INHERITDEBUG(n, o) /* nothing */
#endif /* defined(LOCKDEBUG) */
+/*
+ * Memory barriers.
+ */
+#ifdef __HAVE_ATOMIC_AS_MEMBAR
+#define RW_MEMBAR_ENTER()
+#define RW_MEMBAR_EXIT()
+#define RW_MEMBAR_PRODUCER()
+#else
+#define RW_MEMBAR_ENTER() membar_enter()
+#define RW_MEMBAR_EXIT() membar_exit()
+#define RW_MEMBAR_PRODUCER() membar_producer()
+#endif
+
static void rw_abort(const char *, size_t, krwlock_t *, const char *);
static void rw_dump(const volatile void *, lockop_printer_t);
static lwp_t *rw_owner(wchan_t);
@@ -321,7 +334,7 @@
LOCKSTAT_ENTER(lsflag);
KPREEMPT_DISABLE(curlwp);
- for (owner = rw->rw_owner; ;) {
+ for (owner = rw->rw_owner;;) {
/*
* Read the lock owner field. If the need-to-wait
* indicator is clear, then try to acquire the lock.
@@ -331,7 +344,7 @@
~RW_WRITE_WANTED);
if (__predict_true(next == owner)) {
/* Got it! */
- membar_enter();
+ RW_MEMBAR_ENTER();
break;
}
@@ -460,7 +473,7 @@
* proceed to do direct handoff if there are waiters, and if the
* lock would become unowned.
*/
- membar_exit();
+ RW_MEMBAR_EXIT();
for (;;) {
newown = (owner - decr);
if ((newown & (RW_THREAD | RW_HAS_WAITERS)) == RW_HAS_WAITERS)
@@ -554,13 +567,12 @@
}
for (owner = rw->rw_owner;; owner = next) {
- owner = rw->rw_owner;
if (__predict_false((owner & need_wait) != 0))
return 0;
next = rw_cas(rw, owner, owner + incr);
if (__predict_true(next == owner)) {
/* Got it! */
- membar_enter();
+ RW_MEMBAR_ENTER();
break;
}
}
@@ -576,7 +588,8 @@
/*
* rw_downgrade:
*
- * Downgrade a write lock to a read lock.
+ * Downgrade a write lock to a read lock. Optimise memory accesses for
+ * the uncontended case.
*/
void
rw_downgrade(krwlock_t *rw)
@@ -594,24 +607,20 @@
__USE(curthread);
#endif
-
- membar_producer();
- owner = rw->rw_owner;
- if ((owner & RW_HAS_WAITERS) == 0) {
- /*
- * There are no waiters, so we can do this the easy way.
- * Try swapping us down to one read hold. If it fails, the
- * lock condition has changed and we most likely now have
- * waiters.
- */
- next = rw_cas(rw, owner, RW_READ_INCR);
- if (__predict_true(next == owner)) {
- RW_LOCKED(rw, RW_READER);
- RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0);
- RW_DASSERT(rw, RW_COUNT(rw) != 0);
- return;
- }
- owner = next;
+ /*
+ * If there are no waiters, so we can do this the easy way.
+ * Try swapping us down to one read hold. If it fails, the
+ * lock condition has changed and we most likely now have
+ * waiters.
+ */
+ RW_MEMBAR_PRODUCER();
+ owner = curthread | RW_WRITE_LOCKED;
+ next = rw_cas(rw, owner, RW_READ_INCR);
+ if (__predict_true(next == owner)) {
+ RW_LOCKED(rw, RW_READER);
+ RW_DASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0);
+ RW_DASSERT(rw, RW_COUNT(rw) != 0);
+ return;
}
/*
@@ -619,7 +628,8 @@
* on the sleep queue. Once we have that, we can adjust the
* waiter bits.
*/
- for (;; owner = next) {
+ for (;;) {
+ owner = next;
ts = turnstile_lookup(rw);
RW_DASSERT(rw, ts != NULL);
@@ -670,8 +680,8 @@
/*
* rw_tryupgrade:
*
- * Try to upgrade a read lock to a write lock. We must be the
- * only reader.
+ * Try to upgrade a read lock to a write lock. We must be the only
+ * reader. Optimise memory accesses for the uncontended case.
*/
int
rw_tryupgrade(krwlock_t *rw)
@@ -682,18 +692,18 @@
RW_ASSERT(rw, curthread != 0);
RW_ASSERT(rw, rw_read_held(rw));
- for (owner = rw->rw_owner;; owner = next) {
- RW_ASSERT(rw, (owner & RW_WRITE_LOCKED) == 0);
- if (__predict_false((owner & RW_THREAD) != RW_READ_INCR)) {
- RW_ASSERT(rw, (owner & RW_THREAD) != 0);
- return 0;
- }
+ for (owner = RW_READ_INCR;; owner = next) {
newown = curthread | RW_WRITE_LOCKED | (owner & ~RW_THREAD);
next = rw_cas(rw, owner, newown);
if (__predict_true(next == owner)) {
- membar_producer();
+ RW_MEMBAR_PRODUCER();
break;
}
+ RW_ASSERT(rw, (next & RW_WRITE_LOCKED) == 0);
+ if (__predict_false((next & RW_THREAD) != RW_READ_INCR)) {
+ RW_ASSERT(rw, (next & RW_THREAD) != 0);
+ return 0;
+ }
}
RW_UNLOCKED(rw, RW_READER);
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