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Thu, 21 Sep 2023 05:14:50 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.25]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7BFE8711282; Thu, 21 Sep 2023 05:14:49 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 5408221E6900; Thu, 21 Sep 2023 07:14:48 +0200 (CEST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: ccf161fd-583d-11ee-9b0d-b553b5be7939 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1695273294; h=from:from: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=hx9+KIwAjLK1jnKVdQOUCTEgU0GYm+uRgFTbQHY6/Js=; b=KMUVruviCkBBoY42QtAORbyXay6qBT0cSq2xvgNhkwlWFGGDh2bgdQPMPD6KktbxWZ5SLx 3zo3YLgbOiXqLM57mywoupBkhLz/9IQ+67lfoY7yZNAEv+pp9OMisTTFezGA7h7BJqF/pg O6eUvcDM3f83AiVu8SxULmHkhkMJdjk= X-MC-Unique: f9bjcqYzMn-wyOCa4wcWbg-1 From: Markus Armbruster To: Eric Blake Cc: qemu-devel@nongnu.org, kwolf@redhat.com, hreitz@redhat.com, vsementsov@yandex-team.ru, jsnow@redhat.com, idryomov@gmail.com, pl@kamp.de, sw@weilnetz.de, sstabellini@kernel.org, anthony.perard@citrix.com, paul@xen.org, pbonzini@redhat.com, marcandre.lureau@redhat.com, berrange@redhat.com, thuth@redhat.com, philmd@linaro.org, stefanha@redhat.com, fam@euphon.net, quintela@redhat.com, peterx@redhat.com, leobras@redhat.com, kraxel@redhat.com, qemu-block@nongnu.org, xen-devel@lists.xenproject.org, alex.bennee@linaro.org, peter.maydell@linaro.org Subject: Re: [PATCH v2 7/7] qobject atomics osdep: Make a few macros more hygienic References: <20230920183149.1105333-1-armbru@redhat.com> <20230920183149.1105333-8-armbru@redhat.com> <3gjitlytxqobntg4fluip365s5ijwt3h2zvf3tjbmtiifikv53@xxrl7slw5tjt> Date: Thu, 21 Sep 2023 07:14:48 +0200 In-Reply-To: <3gjitlytxqobntg4fluip365s5ijwt3h2zvf3tjbmtiifikv53@xxrl7slw5tjt> (Eric Blake's message of "Wed, 20 Sep 2023 15:10:58 -0500") Message-ID: <87zg1g9kfr.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 Eric Blake writes: > On Wed, Sep 20, 2023 at 08:31:49PM +0200, Markus Armbruster wrote: > ... >> The only reliable way to prevent unintended variable name capture is >> -Wshadow. >> >> One blocker for enabling it is shadowing hiding in function-like >> macros like >> >> qdict_put(dict, "name", qobject_ref(...)) >> >> qdict_put() wraps its last argument in QOBJECT(), and the last >> argument here contains another QOBJECT(). >> >> Use dark preprocessor sorcery to make the macros that give us this >> problem use different variable names on every call. >> >> Signed-off-by: Markus Armbruster >> Reviewed-by: Eric Blake > > It's changed (for the better) since v1, so I'm re-reviewing. > >> --- >> include/qapi/qmp/qobject.h | 11 +++++++++-- >> include/qemu/atomic.h | 17 ++++++++++++----- >> include/qemu/compiler.h | 3 +++ >> include/qemu/osdep.h | 31 +++++++++++++++++++++++-------- >> 4 files changed, 47 insertions(+), 15 deletions(-) >> >> diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h >> index 9003b71fd3..d36cc97805 100644 >> --- a/include/qapi/qmp/qobject.h >> +++ b/include/qapi/qmp/qobject.h >> @@ -45,10 +45,17 @@ struct QObject { >> struct QObjectBase_ base; >> }; >> >> -#define QOBJECT(obj) ({ \ >> +/* >> + * Preprocessory sorcery ahead: use a different identifier for the > > s/Preprocessory/Preprocessor/ (multiple times in the patch) Dang! Will fix. >> + * local variable in each expansion, so we can nest macro calls >> + * without shadowing variables. >> + */ >> +#define QOBJECT_INTERNAL(obj, _obj) ({ \ >> typeof(obj) _obj = (obj); \ >> - _obj ? container_of(&(_obj)->base, QObject, base) : NULL; \ >> + _obj \ >> + ? container_of(&(_obj)->base, QObject, base) : NULL; \ > > As pointed out before, you can write &_obj->base instead of > &(_obj)->base, now that we know _obj is a single identifier rather > than an arbitrary expression. Not strictly necessary since the extra > () doesn't change semantics... Makes sense, I just forgot here. >> }) >> +#define QOBJECT(obj) QOBJECT_INTERNAL((obj), MAKE_IDENTFIER(_obj)) >> >> /* Required for qobject_to() */ >> #define QTYPE_CAST_TO_QNull QTYPE_QNULL >> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h >> index d95612f7a0..d4cbd01909 100644 >> --- a/include/qemu/atomic.h >> +++ b/include/qemu/atomic.h >> @@ -157,13 +157,20 @@ >> smp_read_barrier_depends(); >> #endif >> >> -#define qatomic_rcu_read(ptr) \ >> - ({ \ >> +/* >> + * Preprocessory sorcery ahead: use a different identifier for the >> + * local variable in each expansion, so we can nest macro calls >> + * without shadowing variables. >> + */ >> +#define qatomic_rcu_read_internal(ptr, _val) \ >> + ({ \ >> qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \ >> - typeof_strip_qual(*ptr) _val; \ >> - qatomic_rcu_read__nocheck(ptr, &_val); \ >> - _val; \ >> + typeof_strip_qual(*ptr) _val; \ >> + qatomic_rcu_read__nocheck(ptr, &_val); \ > > ...but it looks odd for the patch to not be consistent on that front. > >> + _val; \ >> }) >> +#define qatomic_rcu_read(ptr) \ >> + qatomic_rcu_read_internal((ptr), MAKE_IDENTFIER(_val)) >> >> #define qatomic_rcu_set(ptr, i) do { \ >> qemu_build_assert(sizeof(*ptr) <= ATOMIC_REG_SIZE); \ >> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h >> index a309f90c76..03236d830c 100644 >> --- a/include/qemu/compiler.h >> +++ b/include/qemu/compiler.h >> @@ -37,6 +37,9 @@ >> #define tostring(s) #s >> #endif >> >> +/* Expands into an identifier stemN, where N is another number each time */ >> +#define MAKE_IDENTFIER(stem) glue(stem, __COUNTER__) > > I like how this turned out. > > With the spelling fix, and optionally with the redundant () dropped, > you can keep my R-b. Thanks!