Re: [Qemu-devel] [PATCH 3/5] qemu-thread: use acquire/release to clarify semantics of QemuEvent

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@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 <pbonzini@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.

>           */
> @@ -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