On 12/10/2016 11:21, Alex Bennée wrote:
>
> Paolo Bonzini <pbonz...@redhat.com> writes:
>
>> 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 <pbonz...@redhat.com>
>> ---
>> 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.
>
> Why are we saving value here? We never use it.
It should be used in the "if", and patchew rightly complained. I didn't
run the docker-test-mingw compilation on this series.
Paolo
>
>> */
>> @@ -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
>
>
> --
> Alex Bennée
>
