From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 35220CCA470 for ; Wed, 8 Oct 2025 16:18:12 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1v6Wqn-0002sG-Fr; Wed, 08 Oct 2025 12:17:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1v6WqR-0002km-U2 for qemu-devel@nongnu.org; Wed, 08 Oct 2025 12:16:54 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1v6WqJ-0005Ax-TZ for qemu-devel@nongnu.org; Wed, 08 Oct 2025 12:16:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1759940196; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=0CvxHy96HLNAXOe4GJyLHMqVOMpF0iix3m7vM3BSmis=; b=LA/cxDFH9vmgKXSQS509V5CHF8MShNdrrOQZ0Wqx7wJmm6a0Pt1/qZWET651odU+XpJuv3 Fs0T7JrBSJ0CTcmHSvQoYkZ8jLepss/TKTJCvSgiCBJaGjPKaAFOXW1h7ukNyryArlwQCa C9tHFch5JLpvh7hGKNx5LZ1XwU8RMpU= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-258-hjFtaM1JNOqGYtzPwZjOOg-1; Wed, 08 Oct 2025 12:16:32 -0400 X-MC-Unique: hjFtaM1JNOqGYtzPwZjOOg-1 X-Mimecast-MFC-AGG-ID: hjFtaM1JNOqGYtzPwZjOOg_1759940191 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 27CDD180034D for ; Wed, 8 Oct 2025 16:16:31 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.74]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 50116180057B; Wed, 8 Oct 2025 16:16:28 +0000 (UTC) Date: Wed, 8 Oct 2025 17:16:25 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Paolo Bonzini Cc: qemu-devel@nongnu.org, armbru@redhat.com Subject: Re: [PATCH] qobject: make refcount atomic Message-ID: References: <20251008152701.411964-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20251008152701.411964-1-pbonzini@redhat.com> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.442, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, T_SPF_TEMPERROR=0.01 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Oct 08, 2025 at 05:27:01PM +0200, Paolo Bonzini wrote: > The Rust bindings for QObject will only operate on complete objects, > treating them as immutable as long as the Rust QObject is live. > > With that constraint, it is trivial for Rust code to treat QObjects as > thread-safe; all that's needed is to make reference count operations > atomic. Do the same when the C code adds or removes references, since > we don't really know what the Rust code is up to; of course C code will > have to agree with not making changes to the QObjects after they've > been passed to Rust code. I wonder whether that latter constraint on the C code will hold true in practice ? Particularly thinking about scenarios where we add/remove link properties to QObject's, potentially at any time during life of QEMU via QMP/HMP commands ? > > Signed-off-by: Paolo Bonzini > --- > include/qobject/qobject.h | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/include/qobject/qobject.h b/include/qobject/qobject.h > index a6244d0ce00..02f4c6a6eb2 100644 > --- a/include/qobject/qobject.h > +++ b/include/qobject/qobject.h > @@ -32,6 +32,7 @@ > #ifndef QOBJECT_H > #define QOBJECT_H > > +#include "qemu/atomic.h" > #include "qapi/qapi-builtin-types.h" > > /* Not for use outside include/qobject/ */ > @@ -73,7 +74,7 @@ QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7, > static inline void qobject_ref_impl(QObject *obj) > { > if (obj) { > - obj->base.refcnt++; > + qatomic_inc(&obj->base.refcnt); > } > } > > @@ -95,7 +96,7 @@ void qobject_destroy(QObject *obj); > static inline void qobject_unref_impl(QObject *obj) > { > assert(!obj || obj->base.refcnt); > - if (obj && --obj->base.refcnt == 0) { > + if (obj && qatomic_fetch_dec(&obj->base.refcnt) == 1) { > qobject_destroy(obj); > } > } With this unref method being void, how does the Rust code know when a QObject is no longer alive after it has called unref ? Does it need to know this to provide any safety guarantees ? With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|