diff options
| author | Paolo Bonzini <pbonzini@redhat.com> | 2016-09-19 11:10:57 +0200 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2016-10-24 11:30:55 +0200 |
| commit | 374293ca6fb060f0302e5ca76ddccbc2bbb075c9 (patch) | |
| tree | dc95662c0cdcc77ecf1e2f2416756ef05a146f79 /util/qemu-thread-posix.c | |
| parent | f1ee86963b9a7bc6a60b823dbf682fd0a62ffcc4 (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-posix.c')
| -rw-r--r-- | util/qemu-thread-posix.c | 15 |
1 files changed, 12 insertions, 3 deletions
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c index 74a3023f3e..ce51b37c1d 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) { /* |