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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id DDCB5C433FE for ; Fri, 21 Oct 2022 15:42:28 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 528E38E0002; Fri, 21 Oct 2022 11:42:28 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 4D8ED8E0001; Fri, 21 Oct 2022 11:42:28 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 3C7BB8E0002; Fri, 21 Oct 2022 11:42:28 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 2D5A68E0001 for ; Fri, 21 Oct 2022 11:42:28 -0400 (EDT) Received: from smtpin04.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 045FC80493 for ; Fri, 21 Oct 2022 15:42:27 +0000 (UTC) X-FDA: 80045373576.04.D3CEDC0 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf13.hostedemail.com (Postfix) with ESMTP id 5A5432003E for ; Fri, 21 Oct 2022 15:42:26 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 69C6261F11; Fri, 21 Oct 2022 15:42:25 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B3FE2C433B5; Fri, 21 Oct 2022 15:42:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1666366944; bh=ihl4EJMW8MrgXokA2/Iw5cykXpYzofVMTWKlBmW1ZTA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=u1LCnPoCDTG8WUlFA7f/79ns/p/yfpGVN7bshVHcLGS0YD8dZoIarUbksxnTgj2Tc XsHxl8J2UN5VtTnDGgCS7hcbDxn5y3yQKggkyZf+7JP6EJb17xtWsiv3tL5YErcZEP FttZfjAnyno0E8Zxtg8bmXaHa3cJYFbEW8aHjMFpRoMGtX2OqvPKdS/D1fhLD3hIvc We5bGhiGMS7Bu2ym64ZkaFf7GNseosDmFWhJo2WDS+Ax27j9iuh2PfRfX/kjwB1xIU EdaqVb1DzTrrsudSMlrRMi3nFRlgvfHs0ol5q1gO2NIv+XH/5o4AKvcoKgsUto8Umy fYuzhcNP1Lfdw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 5720E5C0543; Fri, 21 Oct 2022 08:42:24 -0700 (PDT) Date: Fri, 21 Oct 2022 08:42:24 -0700 From: "Paul E. McKenney" To: Vlastimil Babka Cc: Christoph Lameter , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org Subject: Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference before locking Message-ID: <20221021154224.GJ5600@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221019224652.GA2499358@paulmck-ThinkPad-P17-Gen-1> <20221019224659.2499511-5-paulmck@kernel.org> <03d5730-9241-542d-76c6-728be4487c4@gentwo.de> <20221021134309.GG5600@paulmck-ThinkPad-P17-Gen-1> <8504f36a-b6bc-4c46-843e-55d8cd5a1dd0@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8504f36a-b6bc-4c46-843e-55d8cd5a1dd0@suse.cz> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666366946; a=rsa-sha256; cv=none; b=74ldvKuaiLV0tEJhpysb6/9Pd/bL1Q+tXy20SyHyQkyiOK6Gpve5jPJ2qlPa6Xn9DMa+AO 5xxbhtGV18rPCzSUk9noeB7gjvynow+CVR0Rf6xV0G5KEBC1TQ3HU92Rl+yDkqMoBeREM/ JQv2BbWAoqOO8RJNTkIVdfjgvEWB8hw= ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=u1LCnPoC; spf=pass (imf13.hostedemail.com: domain of "SRS0=P1bP=2W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 139.178.84.217 as permitted sender) smtp.mailfrom="SRS0=P1bP=2W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666366946; h=from:from:sender:reply-to: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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=1wMmTRVNgRY6mGTiqBQB8HgYjngazqPrTKo1s5AFyxs=; b=ZnXE6D5S8jkFYOGe1S7TuCTGrqmGNPL+tXn8CeCXa3bo4ZkOqzrn3XrJ13S5qPRpGA8wY7 bqk/J1beSGdrmnI2ixFYIsdXbVZlOqi/PE0BpknYq8kk/9n6bwQMmfJfUOPl5sKj0SUNBL i/JO467f67Z81riQkmUg+/288H2nOgs= X-Stat-Signature: kfci31khjaahubgppemdxkp4eh9rnosa X-Rspamd-Queue-Id: 5A5432003E Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=u1LCnPoC; spf=pass (imf13.hostedemail.com: domain of "SRS0=P1bP=2W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 139.178.84.217 as permitted sender) smtp.mailfrom="SRS0=P1bP=2W=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1666366946-367336 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Oct 21, 2022 at 03:50:17PM +0200, Vlastimil Babka wrote: > On 10/21/22 15:43, Paul E. McKenney wrote: > > On Fri, Oct 21, 2022 at 09:44:23AM +0200, Christoph Lameter wrote: > >> On Wed, 19 Oct 2022, Paul E. McKenney wrote: > >> > >> > It is not obvious to the casual user why it is absolutely necessary to > >> > acquire a reference to a SLAB_DESTROY_BY_RCU structure before acquiring > >> > a lock in that structure. Therefore, add a comment explaining this point. > >> > >> Sorry but this is not correct and difficult to comprehend. > >> > >> 1. You do not need a reference to a slab object after it was allocated. > >> Objects must be properly protected by rcu_locks. > >> > >> 2. Locks are initialized once on slab allocation via a constructor (*not* on object allocation via kmem_cache_alloc) > >> > >> 3. Modifying locks at allocation/free is not possible since references to > >> these objects may still persist after free and before alloc. > >> > >> 4. The old term SLAB_DESTROY_BY_RCU is used here. > > > > Thank you for looking this over, but Vlastimil beat you to it. How does > > the update below look? > > LGTM. May I please have your ack? Thanx, Paul > > ------------------------------------------------------------------------ > > > > commit ff4c536e6b44e2e185e38c3653851f92e07139da > > Author: Paul E. McKenney > > Date: Mon Sep 26 08:57:56 2022 -0700 > > > > slab: Explain why SLAB_TYPESAFE_BY_RCU reference before locking > > > > It is not obvious to the casual user why it is absolutely necessary to > > acquire a reference to a SLAB_TYPESAFE_BY_RCU structure before acquiring > > a lock in that structure. Therefore, add a comment explaining this point. > > > > [ paulmck: Apply Vlastimil Babka feedback. ] > > > > Signed-off-by: Paul E. McKenney > > Acked-by: Vlastimil Babka > > > Cc: Christoph Lameter > > Cc: Pekka Enberg > > Cc: David Rientjes > > Cc: Joonsoo Kim > > Cc: Andrew Morton > > Cc: Vlastimil Babka > > Cc: Roman Gushchin > > Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com> > > Cc: > > > > diff --git a/include/linux/slab.h b/include/linux/slab.h > > index 90877fcde70bd..487418c7ea8cd 100644 > > --- a/include/linux/slab.h > > +++ b/include/linux/slab.h > > @@ -76,6 +76,17 @@ > > * rcu_read_lock before reading the address, then rcu_read_unlock after > > * taking the spinlock within the structure expected at that address. > > * > > + * Note that it is not possible to acquire a lock within a structure > > + * allocated with SLAB_TYPESAFE_BY_RCU without first acquiring a reference > > + * as described above. The reason is that SLAB_TYPESAFE_BY_RCU pages > > + * are not zeroed before being given to the slab, which means that any > > + * locks must be initialized after each and every kmem_struct_alloc(). > > + * Alternatively, make the ctor passed to kmem_cache_create() initialize > > + * the locks at page-allocation time, as is done in __i915_request_ctor(), > > + * sighand_ctor(), and anon_vma_ctor(). Such a ctor permits readers > > + * to safely acquire those ctor-initialized locks under rcu_read_lock() > > + * protection. > > + * > > * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU. > > */ > > /* Defer freeing slabs to RCU */ >