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163 lines
6.0 KiB
163 lines
6.0 KiB
From 02347869410fe53d814487501fb586f7dc614375 Mon Sep 17 00:00:00 2001
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From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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Date: Thu, 9 Mar 2023 08:24:36 -0500
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Subject: [PATCH 06/12] qemu-thread-win32: cleanup, fix, document QemuEvent
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RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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RH-MergeRequest: 158: qatomic: add smp_mb__before/after_rmw()
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RH-Bugzilla: 2175660
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
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RH-Acked-by: David Hildenbrand <david@redhat.com>
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RH-Acked-by: Eric Auger <eric.auger@redhat.com>
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RH-Commit: [3/9] d228e9d6a4a75dd1f0a23a6dceaf4fea23d69192 (eesposit/qemu-kvm)
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Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175660
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commit 6c5df4b48f0c52a61342ecb307a43f4c2a3565c4
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Author: Paolo Bonzini <pbonzini@redhat.com>
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Date: Thu Mar 2 11:22:50 2023 +0100
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qemu-thread-win32: cleanup, fix, document QemuEvent
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QemuEvent is currently broken on ARM due to missing memory barriers
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after qatomic_*(). Apart from adding the memory barrier, a closer look
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reveals some unpaired memory barriers that are not really needed and
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complicated the functions unnecessarily. Also, it is relying on
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a memory barrier in ResetEvent(); the barrier _ought_ to be there
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but there is really no documentation about it, so make it explicit.
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Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
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Reviewed-by: David Hildenbrand <david@redhat.com>
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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---
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util/qemu-thread-win32.c | 82 +++++++++++++++++++++++++++-------------
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1 file changed, 56 insertions(+), 26 deletions(-)
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diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
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index 69db254ac7..a7fe3cc345 100644
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--- a/util/qemu-thread-win32.c
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+++ b/util/qemu-thread-win32.c
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@@ -272,12 +272,20 @@ void qemu_event_destroy(QemuEvent *ev)
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void qemu_event_set(QemuEvent *ev)
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{
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assert(ev->initialized);
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- /* qemu_event_set has release semantics, but because it *loads*
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+
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+ /*
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+ * Pairs with both qemu_event_reset() and qemu_event_wait().
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+ *
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+ * qemu_event_set has release semantics, but because it *loads*
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* ev->value we need a full memory barrier here.
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*/
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smp_mb();
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if (qatomic_read(&ev->value) != EV_SET) {
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- if (qatomic_xchg(&ev->value, EV_SET) == EV_BUSY) {
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+ int old = qatomic_xchg(&ev->value, EV_SET);
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+
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+ /* Pairs with memory barrier after ResetEvent. */
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+ smp_mb__after_rmw();
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+ if (old == EV_BUSY) {
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/* There were waiters, wake them up. */
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SetEvent(ev->event);
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}
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@@ -286,17 +294,19 @@ void qemu_event_set(QemuEvent *ev)
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void qemu_event_reset(QemuEvent *ev)
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{
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- unsigned value;
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-
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assert(ev->initialized);
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- value = qatomic_read(&ev->value);
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- smp_mb_acquire();
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- if (value == EV_SET) {
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- /* If there was a concurrent reset (or even reset+wait),
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- * do nothing. Otherwise change EV_SET->EV_FREE.
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- */
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- qatomic_or(&ev->value, EV_FREE);
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- }
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+
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+ /*
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+ * If there was a concurrent reset (or even reset+wait),
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+ * do nothing. Otherwise change EV_SET->EV_FREE.
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+ */
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+ qatomic_or(&ev->value, EV_FREE);
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+
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+ /*
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+ * Order reset before checking the condition in the caller.
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+ * Pairs with the first memory barrier in qemu_event_set().
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+ */
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+ smp_mb__after_rmw();
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}
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void qemu_event_wait(QemuEvent *ev)
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@@ -304,29 +314,49 @@ void qemu_event_wait(QemuEvent *ev)
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unsigned value;
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assert(ev->initialized);
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- value = qatomic_read(&ev->value);
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- smp_mb_acquire();
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+
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+ /*
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+ * qemu_event_wait must synchronize with qemu_event_set even if it does
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+ * not go down the slow path, so this load-acquire is needed that
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+ * synchronizes with the first memory barrier in qemu_event_set().
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+ *
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+ * If we do go down the slow path, there is no requirement at all: we
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+ * might miss a qemu_event_set() here but ultimately the memory barrier in
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+ * qemu_futex_wait() will ensure the check is done correctly.
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+ */
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+ value = qatomic_load_acquire(&ev->value);
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if (value != EV_SET) {
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if (value == EV_FREE) {
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- /* qemu_event_set is not yet going to call SetEvent, but we are
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- * going to do another check for EV_SET below when setting EV_BUSY.
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- * At that point it is safe to call WaitForSingleObject.
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+ /*
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+ * Here the underlying kernel event is reset, but qemu_event_set is
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+ * not yet going to call SetEvent. However, there will be another
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+ * check for EV_SET below when setting EV_BUSY. At that point it
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+ * is safe to call WaitForSingleObject.
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*/
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ResetEvent(ev->event);
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- /* Tell qemu_event_set that there are waiters. No need to retry
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- * because there cannot be a concurrent busy->free transition.
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- * After the CAS, the event will be either set or busy.
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+ /*
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+ * It is not clear whether ResetEvent provides this barrier; kernel
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+ * APIs (KeResetEvent/KeClearEvent) do not. Better safe than sorry!
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+ */
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+ smp_mb();
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+
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+ /*
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+ * Leave the event reset and tell qemu_event_set that there are
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+ * waiters. No need to retry, because there cannot be a concurrent
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+ * busy->free transition. After the CAS, the event will be either
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+ * set or busy.
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*/
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if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
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- value = EV_SET;
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- } else {
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- value = EV_BUSY;
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+ return;
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}
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}
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- if (value == EV_BUSY) {
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- WaitForSingleObject(ev->event, INFINITE);
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- }
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+
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+ /*
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+ * ev->value is now EV_BUSY. Since we didn't observe EV_SET,
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+ * qemu_event_set() must observe EV_BUSY and call SetEvent().
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+ */
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+ WaitForSingleObject(ev->event, INFINITE);
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}
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}
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--
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2.39.1
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