From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTP id AD5D75A026F for ; Fri, 5 Jan 2024 08:55:21 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1704441320; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=OEGSWt0ogetuwi8VF2nRbaNozdsMPM7P7+qR9XiOcNM=; b=AS2J6rbddyh9jHjjmVFiGNqgpSIiSrhdJhB1NCh7dxSPUnA553/erBBJyneUY/X27/czRA /G2U7cN64tiZvz9v2MBvY/gHDl8Fu9lUmM+FomJpq11Hm/p9n1bd7rmkUR3HI9C2PpGeTj 3cdtjZlqRWSotG1El+bkImEyXDJ+RLQ= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-19-vCecwWfNP-SjTRRfTqnaig-1; Fri, 05 Jan 2024 02:55:16 -0500 X-MC-Unique: vCecwWfNP-SjTRRfTqnaig-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id 9EA83832D1A; Fri, 5 Jan 2024 07:55:16 +0000 (UTC) Received: from elisabeth (unknown [10.39.208.16]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E166B2166B31; Fri, 5 Jan 2024 07:55:15 +0000 (UTC) Date: Fri, 5 Jan 2024 08:55:13 +0100 From: Stefano Brivio To: David Gibson Subject: Re: [PATCH v3 10/13] flow: Move flow_count from context structure to a global Message-ID: <20240105085513.5eadf3fc@elisabeth> In-Reply-To: References: <20231221061549.976358-1-david@gibson.dropbear.id.au> <20231221061549.976358-11-david@gibson.dropbear.id.au> <20231228192459.312cc508@elisabeth> <20240102191335.413b2b04@elisabeth> <20240103080834.24fa0a7a@elisabeth> Organization: Red Hat MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.6 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Message-ID-Hash: D2Q4ETFT2KQF3ZCPR6L4PAJA6P5HNAJT X-Message-ID-Hash: D2Q4ETFT2KQF3ZCPR6L4PAJA6P5HNAJT X-MailFrom: sbrivio@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Thu, 4 Jan 2024 20:51:19 +1100 David Gibson wrote: > On Wed, Jan 03, 2024 at 08:08:34AM +0100, Stefano Brivio wrote: > > On Wed, 3 Jan 2024 14:54:27 +1100 > > David Gibson wrote: > > =20 > > > I'm not sure where to get the actual text of the standards =20 > >=20 > > Let me answer this first: one (the?) trick is to use so-called final > > drafts, which are made freely available (same as working drafts) by the > > Working Group. > >=20 > > Those are not the same as the standards, but differences from the final > > draft are also published... and they are usually not substantial. > >=20 > > This is _very_ informative: > > https://stackoverflow.com/questions/81656/where-do-i-find-the-current= -c-or-c-standard-documents =20 >=20 > Ah, thanks. >=20 > > Wikipedia also has the links, by the way. Anyway, in practice: > >=20 > > - C11: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf > > - C99: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf > > - C89: > > https://web.archive.org/web/20200909074736if_/https://www.pdf-archive= .com/2014/10/02/ansi-iso-9899-1990-1/ansi-iso-9899-1990-1.pdf > > =20 > > > On Tue, Jan 02, 2024 at 07:13:35PM +0100, Stefano Brivio wrote: =20 > > > > On Sun, 31 Dec 2023 16:58:39 +1100 > > > > David Gibson wrote: > > > > =20 > > > > > On Thu, Dec 28, 2023 at 07:25:18PM +0100, Stefano Brivio wrote: = =20 > > > > > > On Thu, 21 Dec 2023 17:15:46 +1100 > > > > > > David Gibson wrote: > > > > > > =20 > > > > > > > In general, the passt code is a bit haphazard about what's a = true global > > > > > > > variable and what's in the quasi-global 'context structure'. = The > > > > > > > flow_count field is one such example: it's in the context str= ucture, > > > > > > > although it's really part of the same data structure as flowt= ab[], which > > > > > > > is a genuine global. =20 > > > > > >=20 > > > > > > Well, the reason is that flow_tab[FLOW_MAX] might be problemati= cally > > > > > > too big to live on the stack, unlike flow_count. > > > > > >=20 > > > > > > But anyway, as far as thoughts of multithreading are concerned,= both > > > > > > should probably be global. And sure, it's more consistent this = way. > > > > > > =20 > > > > > > > Move flow_count to be a regular global to match. For now it = needs to be > > > > > > > public, rather than static, but we expect to be able to chang= e that in > > > > > > > future. =20 > > > > > >=20 > > > > > > If it's not static, it should be initialised, and that's not do= ne here. =20 > > > > >=20 > > > > > Uh... what? "static" here is meaning module-global rather than > > > > > global-global, which has no bearing on initialisation. AFAIK glo= bals > > > > > are zero-initialised whether they're static or not. =20 > > > >=20 > > > > ...and to my utter surprise, I just discovered that if you talk C11= , > > > > you're right. From the N1570 draft (ISO/IEC 9899:201x), Section 6.7= .9 > > > > "Initialization", clause 10: > > > >=20 > > > > If an object that has automatic storage duration is not initializ= ed > > > > explicitly, its value is indeterminate. If an object that has sta= tic > > > > or thread storage duration is not initialized explicitly, then: > > > >=20 > > > > [...] > > > >=20 > > > > =E2=80=94 if it has arithmetic type, it is initialized to (positi= ve or > > > > unsigned) zero; > > > >=20 > > > > And 'flow_count' has thread storage duration. =20 > > >=20 > > > No.. I don't think it does. AFAICT only thread-local variables have > > > thread storage duration. As a global flow_count will have static > > > storage duration, even without the static keyword. =20 > >=20 > > So, C11 defines static storage duration here: > >=20 > > 6.2.4 Storage durations of objects > >=20 > > [...] > >=20 > > 3 An object whose identifier is declared without the storage-class > > specifier _Thread_local, and either with external or internal linkage > > or with the storage-class specifier static, has static storage > > duration. Its lifetime is the entire execution of the program and its > > stored value is initialized only once, prior to program startup. > >=20 > > do we have any linkage here? I would have said no -- but, going back > > to C99 for this, "6.2.2 Linkages of identifiers": > >=20 > > 5 [...] If the declaration of an identifier for an object has file > > scope and no storage-class specifier, its linkage is external. > >=20 > > which supports your paragraph below. =20 >=20 > Right. >=20 > > By the way, C11 now says: > >=20 > > 6.11.2 Linkages of identifiers > >=20 > > 1 Declaring an identifier with internal linkage at file scope without > > the static storage-class specifier is an obsolescent feature =20 >=20 > Ok. I'm not even sure how you would do that. By doing what I *thought* you were doing (see below): "int x" at file scope (outside functions), no static, no extern declaration, nothing. > > > > In C99, however (draft > > > > N1256), Section 6.7.8 "Initialization", clause 10: > > > >=20 > > > > If an object that has automatic storage duration is not initializ= ed > > > > explicitly, its value is indeterminate. If an object that has sta= tic > > > > storage duration is not initialized explicitly, then: > > > >=20 > > > > [...] > > > >=20 > > > > note the missing "or thread storage duration". > > > >=20 > > > > C89, the one I was actually basing my observation on, says, at 3.5.= 7 > > > > "Initialization": > > > >=20 > > > > If an object that has static storage duration is not initialized > > > > explicitly, it is initialized implicitly as if every member that = has > > > > arithmetic type were assigned 0 and every member that has pointer= type > > > > were assigned a null pointer constant. If an object that has > > > > automatic storage duration is not initialized explicitly, its val= ue is > > > > indeterminate. > > > >=20 > > > > so... um. We won't go back to C99. But to me, and maybe others, not > > > > having a "=3D 0;" for a "global" means pretty much that we don't re= ly on > > > > any particular initial value. =20 > > >=20 > > > Again, I'm pretty sure that's not true, even for C99 and C89. AIUI, > > > 'static' locals and *all* globals have "static storage diration". > > >=20 > > > I'm not sure where to get the actual text of the standards but see fo= r > > > example > > >=20 > > > https://en.cppreference.com/w/c/language/static_storage_duration > > >=20 > > > Here 'flow_count' has external linkage, thus satisfying the condition= s > > > for static storage duration. =20 > >=20 > > Right. Well, for C99 and C11 at least. For C89 things are slightly > > different: > >=20 > > 6.1.2.4 Storage durations of objects > >=20 > > [...] > >=20 > > An object whose identifier is declared with external or internal > > linkage. or with the storage-class specifier static has static storag= e > > duration. > >=20 > > [...] > >=20 > > An object whose identifier is declared with no linkage and without th= e > > storage-class specifier static has automatic storage duration. > >=20 > > You might say it has external linkage. But it was not *declared with* > > external linkage -- it just happens to have it (C89 and C99 don't > > differ here). =20 >=20 > Hrm. We do have: > =09extern unsigned flow_first_free; > in flow_table.h. Does that cound as declaring with external linkage? Gosh, sorry... yes, it also counts as me completely missing it. > > > Fwiw, I'm pretty sure the kernel has relied on zero-initialization of > > > non-static globals for many years. =20 > >=20 > > True, and the opposite is even considered as a style issue since 2007, > > commit f0a594c1c74f ("update checkpatch.pl to version 0.08"). I also > > found a discussion similar to this one: > > https://lore.kernel.org/all/20201102184147.GA42288@localhost/#r > >=20 > > Anyway... a couple of years before that, it must have been a gcc versio= n > > in the late 2.x, I actually hit an issue with it. Was it a compiler > > issue, or the correct interpretation of C89? Or maybe something on the > > lines of: > > https://www.thegoodpenguin.co.uk/blog/u-boot-relocation-bss-hang/ =20 >=20 > If it was an embedded setup, that last one is certainly possible. > Zeroing the BSS is typically the loader's job, and I've certainly seen > loader implementations - particularly in embedded firmware - that got > this wrong. Embedded setup, yes, but in a Linux kernel (early 2.4.x). No 'extern' there, though. --=20 Stefano