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[Qemu-devel] [PATCH 3/5] qemu-thread: use acquire/release to clarify sem
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 3/5] qemu-thread: use acquire/release to clarify semantics of QemuEvent |
Date: |
Mon, 10 Oct 2016 15:59:05 +0200 |
Do not use the somewhat mysterious atomic_mb_read/atomic_mb_set,
instead make sure that the operations on QemuEvent are annotated
with the desired acquire and release semantics.
In particular, qemu_event_set wakes up the waiting thread, so it must
be a release from the POV of the waker (compare with qemu_mutex_unlock).
And it actually needs a full barrier, because that's the only thing that
provides something like a "load-release".
Use smp_mb_acquire until we have atomic_load_acquire and
atomic_store_release in atomic.h.
Signed-off-by: Paolo Bonzini <address@hidden>
---
util/qemu-thread-posix.c | 15 ++++++++++++---
util/qemu-thread-win32.c | 15 ++++++++++++---
2 files changed, 24 insertions(+), 6 deletions(-)
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 74a3023..ce51b37 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -360,7 +360,11 @@ void qemu_event_destroy(QemuEvent *ev)
void qemu_event_set(QemuEvent *ev)
{
- if (atomic_mb_read(&ev->value) != EV_SET) {
+ /* qemu_event_set has release semantics, but because it *loads*
+ * ev->value we need a full memory barrier here.
+ */
+ smp_mb();
+ if (atomic_read(&ev->value) != EV_SET) {
if (atomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
/* There were waiters, wake them up. */
futex_wake(ev, INT_MAX);
@@ -370,7 +374,11 @@ void qemu_event_set(QemuEvent *ev)
void qemu_event_reset(QemuEvent *ev)
{
- if (atomic_mb_read(&ev->value) == EV_SET) {
+ unsigned value;
+
+ value = atomic_read(&ev->value);
+ smp_mb_acquire();
+ if (value == EV_SET) {
/*
* If there was a concurrent reset (or even reset+wait),
* do nothing. Otherwise change EV_SET->EV_FREE.
@@ -383,7 +391,8 @@ void qemu_event_wait(QemuEvent *ev)
{
unsigned value;
- value = atomic_mb_read(&ev->value);
+ value = atomic_read(&ev->value);
+ smp_mb_acquire();
if (value != EV_SET) {
if (value == EV_FREE) {
/*
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index 98a5ddf..dcdc014 100644
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -274,7 +274,11 @@ void qemu_event_destroy(QemuEvent *ev)
void qemu_event_set(QemuEvent *ev)
{
- if (atomic_mb_read(&ev->value) != EV_SET) {
+ /* qemu_event_set has release semantics, but because it *loads*
+ * ev->value we need a full memory barrier here.
+ */
+ smp_mb();
+ if (atomic_read(&ev->value) != EV_SET) {
if (atomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
/* There were waiters, wake them up. */
SetEvent(ev->event);
@@ -284,7 +288,11 @@ void qemu_event_set(QemuEvent *ev)
void qemu_event_reset(QemuEvent *ev)
{
- if (atomic_mb_read(&ev->value) == EV_SET) {
+ unsigned value;
+
+ value = atomic_read(&ev->value);
+ smp_mb_acquire();
+ if (atomic_read(&ev->value) == EV_SET) {
/* If there was a concurrent reset (or even reset+wait),
* do nothing. Otherwise change EV_SET->EV_FREE.
*/
@@ -296,7 +304,8 @@ void qemu_event_wait(QemuEvent *ev)
{
unsigned value;
- value = atomic_mb_read(&ev->value);
+ value = atomic_read(&ev->value);
+ smp_mb_acquire();
if (value != EV_SET) {
if (value == EV_FREE) {
/* qemu_event_set is not yet going to call SetEvent, but we are
--
2.7.4