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bh=QsXpqg/37aKlZIvkdFb2+sFW5FG0DI77hACwT+DgyPI=; b=cMiVptFt+eBCd9BXbuLXqTJH+sMsfoCMPUaQc0i3PstWW0qd6WzdpEBQ56WNpt14OfblfH p4IbQElH3QgYOaXNcESjnXOnDWMuiaaUMQ5+8t02Y3UcI5/EbvZ92wwffQPBZbq6An9Ip2 Eku6ZPIn83MSvtluidmQiZaSWcNsvW8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-114-Tj3NyIYvPWuQsc17ngCOAQ-1; Wed, 30 Sep 2020 10:31:33 -0400 X-MC-Unique: Tj3NyIYvPWuQsc17ngCOAQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id EC2AF1891E93 for ; Wed, 30 Sep 2020 14:31:31 +0000 (UTC) Received: from starship (unknown [10.35.206.29]) by smtp.corp.redhat.com (Postfix) with ESMTP id C442C619B5; Wed, 30 Sep 2020 14:31:27 +0000 (UTC) Message-ID: <9182aa173b31989e07668194e18f4dcb31981388.camel@redhat.com> Subject: Re: [PATCH 06/10] device-core: use atomic_set on .realized property From: Maxim Levitsky To: Paolo Bonzini , qemu-devel@nongnu.org Date: Wed, 30 Sep 2020 17:31:26 +0300 In-Reply-To: <20200925172604.2142227-7-pbonzini@redhat.com> References: <20200925172604.2142227-1-pbonzini@redhat.com> <20200925172604.2142227-7-pbonzini@redhat.com> User-Agent: Evolution 3.36.3 (3.36.3-1.fc32) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mlevitsk@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=mlevitsk@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/30 00:31:59 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.469, 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_H5=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, 2020-09-25 at 13:26 -0400, Paolo Bonzini wrote: > From: Maxim Levitsky > > Some code might race with placement of new devices on a bus. > We currently first place a (unrealized) device on the bus > and then realize it. > > As a workaround, users that scan the child device list, can > check the realized property to see if it is safe to access such a device. > Use an atomic write here too to aid with this. > > A separate discussion is what to do with devices that are unrealized: > It looks like for this case we only call the hotplug handler's unplug > callback and its up to it to unrealize the device. > An atomic operation doesn't cause harm for this code path though. > > Signed-off-by: Maxim Levitsky > Reviewed-by: Stefan Hajnoczi > Message-Id: <20200913160259.32145-6-mlevitsk@redhat.com> > Signed-off-by: Paolo Bonzini > --- > hw/core/qdev.c | 19 ++++++++++++++++++- > include/hw/qdev-core.h | 3 +++ > 2 files changed, 21 insertions(+), 1 deletion(-) > > diff --git a/hw/core/qdev.c b/hw/core/qdev.c > index 59e5e710b7..fc4daa36fa 100644 > --- a/hw/core/qdev.c > +++ b/hw/core/qdev.c > @@ -946,7 +946,25 @@ static void device_set_realized(Object *obj, bool value, Error **errp) > } > } > > + qatomic_store_release(&dev->realized, value); > + > } else if (!value && dev->realized) { > + > + /* > + * Change the value so that any concurrent users are aware > + * that the device is going to be unrealized > + * > + * TODO: change .realized property to enum that states > + * each phase of the device realization/unrealization > + */ > + > + qatomic_set(&dev->realized, value); > + /* > + * Ensure that concurrent users see this update prior to > + * any other changes done by unrealize. > + */ > + smp_wmb(); I''l probably never fully understand where to use read/write/full barrier. If I understand corrctly, read barrier prevents reads done by this thread to be reordered, by the CPU and write barrier prevents writes done by this CPU to be re-ordered. Both (depending on the macro) usually imply compiler barrier (to avoid compilier re-ordering stuff...) > + > QLIST_FOREACH(bus, &dev->child_bus, sibling) { > qbus_unrealize(bus); > } > @@ -961,7 +979,6 @@ static void device_set_realized(Object *obj, bool value, Error **errp) > } > > assert(local_err == NULL); > - dev->realized = value; > return; > > child_realize_fail: > diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h > index 8067497074..39490e76ee 100644 > --- a/include/hw/qdev-core.h > +++ b/include/hw/qdev-core.h > @@ -163,6 +163,9 @@ struct NamedClockList { > /** > * DeviceState: > * @realized: Indicates whether the device has been fully constructed. > + * When accessed without the iothread mutex, consider using > + * qatomic_load_acquire() before accessing any other field in > + * the device. This sounds way better that what I wrote. As you probably noticed recently (but not this patchset yet) I started to review the spelling/comments more throughfully, so hopefully we see less of stuff that I myself read and wonder why I did that many spelling/grammar mistakes.... :-) > * @reset: ResettableState for the device; handled by Resettable interface. > * > * This structure should not be accessed directly. We declare it here Best regards, Maxim Levitsky