From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38079) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btb76-0001HE-8m for qemu-devel@nongnu.org; Mon, 10 Oct 2016 09:59:33 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1btb75-00088c-8Q for qemu-devel@nongnu.org; Mon, 10 Oct 2016 09:59:32 -0400 Received: from mail-qk0-x243.google.com ([2607:f8b0:400d:c09::243]:33176) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1btb75-00088K-3P for qemu-devel@nongnu.org; Mon, 10 Oct 2016 09:59:31 -0400 Received: by mail-qk0-x243.google.com with SMTP id n66so6891906qkf.0 for ; Mon, 10 Oct 2016 06:59:30 -0700 (PDT) Sender: Paolo Bonzini From: Paolo Bonzini Date: Mon, 10 Oct 2016 15:59:05 +0200 Message-Id: <1476107947-31430-4-git-send-email-pbonzini@redhat.com> In-Reply-To: <1476107947-31430-1-git-send-email-pbonzini@redhat.com> References: <1476107947-31430-1-git-send-email-pbonzini@redhat.com> Subject: [Qemu-devel] [PATCH 3/5] qemu-thread: use acquire/release to clarify semantics of QemuEvent List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: cota@braap.org, alex.bennee@linaro.org 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..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. */ @@ -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