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authorPaolo Bonzini <pbonzini@redhat.com>2016-09-19 11:10:57 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2016-10-24 11:30:55 +0200
commit374293ca6fb060f0302e5ca76ddccbc2bbb075c9 (patch)
treedc95662c0cdcc77ecf1e2f2416756ef05a146f79 /util/qemu-thread-win32.c
parentf1ee86963b9a7bc6a60b823dbf682fd0a62ffcc4 (diff)
qemu-thread: use acquire/release to clarify semantics of QemuEvent
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 <pbonzini@redhat.com>
Diffstat (limited to 'util/qemu-thread-win32.c')
-rw-r--r--util/qemu-thread-win32.c15
1 files changed, 12 insertions, 3 deletions
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index 98a5ddff82..072806f792 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 (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