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Tue, 07 Mar 2023 03:43:37 -0800 (PST) X-Google-Smtp-Source: AK7set8gvttryamZCxhsmrPrHfNGQe63XL/I6fg56HOUH7mC/l7HaIs1jrcZ2PQ/yPP4evumfo2Jzw== X-Received: by 2002:a17:907:dab:b0:8ae:11ca:81de with SMTP id go43-20020a1709070dab00b008ae11ca81demr17428733ejc.34.1678189416911; Tue, 07 Mar 2023 03:43:36 -0800 (PST) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: Richard Henderson , David Hildenbrand Subject: [PULL 2/9] qemu-thread-posix: cleanup, fix, document QemuEvent Date: Tue, 7 Mar 2023 12:43:22 +0100 Message-Id: <20230307114329.158094-3-pbonzini@redhat.com> X-Mailer: git-send-email 2.39.1 In-Reply-To: <20230307114329.158094-1-pbonzini@redhat.com> References: <20230307114329.158094-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer2=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer2=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer2=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1678189472425100002 Content-Type: text/plain; charset="utf-8" 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 Reviewed-by: David Hildenbrand Signed-off-by: Paolo Bonzini --- 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 93d250579741..b2e26e21205b 100644 --- a/util/qemu-thread-posix.c +++ b/util/qemu-thread-posix.c @@ -384,13 +384,21 @@ void qemu_event_destroy(QemuEvent *ev) =20 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) !=3D EV_SET) { - if (qatomic_xchg(&ev->value, EV_SET) =3D=3D EV_BUSY) { + int old =3D qatomic_xchg(&ev->value, EV_SET); + + /* Pairs with memory barrier in kernel futex_wait system call. */ + smp_mb__after_rmw(); + if (old =3D=3D EV_BUSY) { /* There were waiters, wake them up. */ qemu_futex_wake(ev, INT_MAX); } @@ -399,18 +407,19 @@ void qemu_event_set(QemuEvent *ev) =20 void qemu_event_reset(QemuEvent *ev) { - unsigned value; - assert(ev->initialized); - value =3D qatomic_read(&ev->value); - smp_mb_acquire(); - if (value =3D=3D 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(); } =20 void qemu_event_wait(QemuEvent *ev) @@ -418,20 +427,40 @@ void qemu_event_wait(QemuEvent *ev) unsigned value; =20 assert(ev->initialized); - value =3D 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 barrie= r in + * qemu_futex_wait() will ensure the check is done correctly. + */ + value =3D qatomic_load_acquire(&ev->value); if (value !=3D EV_SET) { if (value =3D=3D 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 concu= rrent + * busy->free transition. After the CAS, the event will be ei= ther + * set or busy. + * + * This cmpxchg doesn't have particular ordering requirements = if it + * succeeds (moving the store earlier can only cause qemu_even= t_set() + * to issue _more_ wakeups), the failing case needs acquire se= mantics + * like the load above. */ if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) =3D=3D EV_SE= T) { 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); } } --=20 2.39.1