From 374293ca6fb060f0302e5ca76ddccbc2bbb075c9 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Mon, 19 Sep 2016 11:10:57 +0200 Subject: [PATCH] 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 --- 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..072806f 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 -- 2.7.4