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 X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3D461C49ED6 for ; Wed, 11 Sep 2019 17:11:15 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 16272206A5 for ; Wed, 11 Sep 2019 17:11:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 16272206A5 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:53994 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i869C-00048u-65 for qemu-devel@archiver.kernel.org; Wed, 11 Sep 2019 13:11:14 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:47143) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i868Y-0003ST-9V for qemu-devel@nongnu.org; Wed, 11 Sep 2019 13:10:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i868W-0005wx-VN for qemu-devel@nongnu.org; Wed, 11 Sep 2019 13:10:34 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58550) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i868W-0005wL-NY for qemu-devel@nongnu.org; Wed, 11 Sep 2019 13:10:32 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 075513B738 for ; Wed, 11 Sep 2019 17:10:32 +0000 (UTC) Received: from work-vm (ovpn-117-243.ams2.redhat.com [10.36.117.243]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 01B0F60872; Wed, 11 Sep 2019 17:10:30 +0000 (UTC) Date: Wed, 11 Sep 2019 18:10:28 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Message-ID: <20190911171028.GI2894@work-vm> References: <20190911164202.31136-1-dgilbert@redhat.com> <20190911164202.31136-2-dgilbert@redhat.com> <20190911165627.GG24295@redhat.com> <20190911170423.GH2894@work-vm> <20190911170933.GJ24295@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20190911170933.GJ24295@redhat.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Wed, 11 Sep 2019 17:10:32 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH 1/3] rcu: Add automatically released rcu_read_lock variant 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: pbonzini@redhat.com, qemu-devel@nongnu.org, quintela@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrang=E9 (berrange@redhat.com) wrote: > On Wed, Sep 11, 2019 at 06:04:23PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrang=E9 (berrange@redhat.com) wrote: > > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert (g= it) wrote: > > > > From: "Dr. David Alan Gilbert" > > > >=20 > > > > RCU_READ_LOCK_AUTO takes the rcu_read_lock and then uses glib's > > > > g_auto infrastrcture (and thus whatever the compilers hooks are) = to > > > > release it on all exits of the block. > > > >=20 > > > > Note this macro has a variable declaration in, and hence is not i= n > > > > a while loop. > > > >=20 > > > > Signed-off-by: Dr. David Alan Gilbert > > > > --- > > > > include/qemu/rcu.h | 12 ++++++++++++ > > > > 1 file changed, 12 insertions(+) > > > >=20 > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > > > index 22876d1428..6a25b27d28 100644 > > > > --- a/include/qemu/rcu.h > > > > +++ b/include/qemu/rcu.h > > > > @@ -154,6 +154,18 @@ extern void call_rcu1(struct rcu_head *head,= RCUCBFunc *func); > > > > }), = \ > > > > (RCUCBFunc *)g_free); > > > > =20 > > > > +typedef char rcu_read_auto_t; > > > > +static inline void rcu_read_auto_unlock(rcu_read_auto_t *r) > > > > +{ > > > > + rcu_read_unlock(); > > > > +} > > > > + > > > > +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(rcu_read_auto_t, rcu_read_auto_= unlock) > > > > > > > > +#define RCU_READ_LOCK_AUTO g_auto(rcu_read_auto_t) \ > > > > + _rcu_read_auto =3D 'x'; \ > > > > + rcu_read_lock(); > > > > + > > >=20 > > > Functionally this works, but my gut feeling would be to follow > > > the design of GMutexLocker as-is: > > >=20 > > > https://developer.gnome.org/glib/stable/glib-Threads.html#g-mutex= -locker-new > > >=20 > > > so you get a use pattern of > > >=20 > > > g_autoptr(rcu_read_locker) locker =3D rcu_read_locker_new(); > > >=20 > > > This makes it explicit that the code is creating a variable here, w= hich > > > in turns means it is clear to force unlock early with > > >=20 > > > g_clear_pointer(&locker, rcu_read_locker_free) > >=20 > > The difference compared to the g-mutex-locker is that I don't have > > another object to use as my pointer; that uses the address of the GMu= tex > > as the dummy pointer value. I did try an experiment with g_autoptr > > and found that it did need to return a non-NULL value for it to work, > > which then lead me to think what value to use - while it seems to wor= k > > if I return (void *)1 it makes me nervous. >=20 > Yeah, '(void*)1' would have been what I'd pick. The only thing that the > value is used for is to pass to the rcu_read_locker_free() function > which ignores it, which seems safe enough. glib seems to be at least checking it; if you pass NULL the free'r doesn't get called; so it worries me that we'd be relying on the current definition. Dave > Regards, > Daniel > --=20 > |: https://berrange.com -o- https://www.flickr.com/photos/dberr= ange :| > |: https://libvirt.org -o- https://fstop138.berrange= .com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberr= ange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK