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=FROM_EXCESS_BASE64, 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 CCD67C49ED6 for ; Wed, 11 Sep 2019 17:17:05 +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 9B7F82053B for ; Wed, 11 Sep 2019 17:17:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9B7F82053B 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]:54042 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i86Eq-0006zx-OT for qemu-devel@archiver.kernel.org; Wed, 11 Sep 2019 13:17:04 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:48178) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i86E7-0006EU-JC for qemu-devel@nongnu.org; Wed, 11 Sep 2019 13:16:20 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i86E6-0000Ca-80 for qemu-devel@nongnu.org; Wed, 11 Sep 2019 13:16:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60456) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i86E6-0000C7-04 for qemu-devel@nongnu.org; Wed, 11 Sep 2019 13:16:18 -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 58B32369AC for ; Wed, 11 Sep 2019 17:16:17 +0000 (UTC) Received: from redhat.com (unknown [10.42.17.64]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 717F260852; Wed, 11 Sep 2019 17:16:16 +0000 (UTC) Date: Wed, 11 Sep 2019 18:16:14 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: "Dr. David Alan Gilbert" Message-ID: <20190911171614.GK24295@redhat.com> 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> <20190911171028.GI2894@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190911171028.GI2894@work-vm> 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 17:16:17 +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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= 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" On Wed, Sep 11, 2019 at 06:10:28PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrang=C3=A9 (berrange@redhat.com) wrote: > > On Wed, Sep 11, 2019 at 06:04:23PM +0100, Dr. David Alan Gilbert wrot= e: > > > * Daniel P. Berrang=C3=A9 (berrange@redhat.com) wrote: > > > > On Wed, Sep 11, 2019 at 05:42:00PM +0100, Dr. David Alan Gilbert = (git) 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= in > > > > > 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 *hea= d, 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_aut= o_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-mut= ex-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,= which > > > > 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 G= Mutex > > > 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 wor= k, > > > which then lead me to think what value to use - while it seems to w= ork > > > if I return (void *)1 it makes me nervous. > >=20 > > Yeah, '(void*)1' would have been what I'd pick. The only thing that t= he > > value is used for is to pass to the rcu_read_locker_free() function > > which ignores it, which seems safe enough. >=20 > 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 curren= t > definition. This NULL check is part of the API semantics defined for G_DEFINE_AUTO_CLEANUO_FREE_FUNC. It lets you define what the "empty" value is, typically 'NULL', but in fact you don't need to use a pointer type at all. You can use an 'int', for example, and declare that '-1' is your "empty" value: https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html#= G-DEFINE-AUTO-CLEANUP-FREE-FUNC:CAPS Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|