From d46ca52c3f42add549bd3790a41d06594821334e Mon Sep 17 00:00:00 2001

From: Emanuele Giuseppe Esposito <eesposit@redhat.com>

Date: Thu, 9 Mar 2023 08:10:57 -0500

Subject: [PATCH 03/13] qemu-thread-posix: cleanup, fix, document QemuEvent

RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>

RH-MergeRequest: 263: qatomic: add smp_mb__before/after_rmw()

RH-Bugzilla: 2168472

RH-Acked-by: Cornelia Huck <cohuck@redhat.com>

RH-Acked-by: Eric Auger <eric.auger@redhat.com>

RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

RH-Acked-by: David Hildenbrand <david@redhat.com>

RH-Commit: [3/10] 746070c4d78c7f0a9ac4456d9aee69475acb8964

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472

commit 9586a1329f5dce6c1d7f4de53cf0536644d7e593

Author: Paolo Bonzini <pbonzini@redhat.com>

Date:   Thu Mar 2 11:19:52 2023 +0100

    qemu-thread-posix: cleanup, fix, document QemuEvent

    QemuEvent is currently broken on ARM due to missing memory barriers

    after qatomic_*().  Apart from adding the memory barrier, a closer look

    reveals some unpaired memory barriers too.  Document more clearly what

    is going on.

    Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

    Reviewed-by: David Hildenbrand <david@redhat.com>

    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

---

 util/qemu-thread-posix.c | 69 ++++++++++++++++++++++++++++------------

 1 file changed, 49 insertions(+), 20 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c

index e1225b63bd..dd3b6d4670 100644

--- a/util/qemu-thread-posix.c

+++ b/util/qemu-thread-posix.c

@@ -430,13 +430,21 @@ void qemu_event_destroy(QemuEvent *ev)

 void qemu_event_set(QemuEvent *ev)

 {

-    /* qemu_event_set has release semantics, but because it *loads*

+    assert(ev->initialized);

+

+    /*

+     * Pairs with both qemu_event_reset() and qemu_event_wait().

+     *

+     * qemu_event_set has release semantics, but because it *loads*

      * ev->value we need a full memory barrier here.

      */

-    assert(ev->initialized);

     smp_mb();

     if (qatomic_read(&ev->value) != EV_SET) {

-        if (qatomic_xchg(&ev->value, EV_SET) == EV_BUSY) {

+        int old = qatomic_xchg(&ev->value, EV_SET);

+

+        /* Pairs with memory barrier in kernel futex_wait system call.  */

+        smp_mb__after_rmw();

+        if (old == EV_BUSY) {

             /* There were waiters, wake them up.  */

             qemu_futex_wake(ev, INT_MAX);

         }

@@ -445,18 +453,19 @@ void qemu_event_set(QemuEvent *ev)

 void qemu_event_reset(QemuEvent *ev)

 {

-    unsigned value;

-

     assert(ev->initialized);

-    value = qatomic_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.

-         */

-        qatomic_or(&ev->value, EV_FREE);

-    }

+

+    /*

+     * If there was a concurrent reset (or even reset+wait),

+     * do nothing.  Otherwise change EV_SET->EV_FREE.

+     */

+    qatomic_or(&ev->value, EV_FREE);

+

+    /*

+     * Order reset before checking the condition in the caller.

+     * Pairs with the first memory barrier in qemu_event_set().

+     */

+    smp_mb__after_rmw();

 }

 void qemu_event_wait(QemuEvent *ev)

@@ -464,20 +473,40 @@ void qemu_event_wait(QemuEvent *ev)

     unsigned value;

     assert(ev->initialized);

-    value = qatomic_read(&ev->value);

-    smp_mb_acquire();

+

+    /*

+     * qemu_event_wait must synchronize with qemu_event_set even if it does

+     * not go down the slow path, so this load-acquire is needed that

+     * synchronizes with the first memory barrier in qemu_event_set().

+     *

+     * If we do go down the slow path, there is no requirement at all: we

+     * might miss a qemu_event_set() here but ultimately the memory barrier in

+     * qemu_futex_wait() will ensure the check is done correctly.

+     */

+    value = qatomic_load_acquire(&ev->value);

     if (value != EV_SET) {

         if (value == EV_FREE) {

             /*

-             * Leave the event reset and tell qemu_event_set that there

-             * are waiters.  No need to retry, because there cannot be

-             * a concurrent busy->free transition.  After the CAS, the

-             * event will be either set or busy.

+             * Leave the event reset and tell qemu_event_set that there are

+             * waiters.  No need to retry, because there cannot be a concurrent

+             * busy->free transition.  After the CAS, the event will be either

+             * set or busy.

+             *

+             * This cmpxchg doesn't have particular ordering requirements if it

+             * succeeds (moving the store earlier can only cause qemu_event_set()

+             * to issue _more_ wakeups), the failing case needs acquire semantics

+             * like the load above.

              */

             if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {

                 return;

             }

         }

+

+        /*

+         * This is the final check for a concurrent set, so it does need

+         * a smp_mb() pairing with the second barrier of qemu_event_set().

+         * The barrier is inside the FUTEX_WAIT system call.

+         */

         qemu_futex_wait(ev, EV_BUSY);

     }

 }

-- 

2.37.3