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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E4654C433FE for ; Fri, 21 Oct 2022 15:52:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229779AbiJUPwv (ORCPT ); Fri, 21 Oct 2022 11:52:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55502 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230243AbiJUPwn (ORCPT ); Fri, 21 Oct 2022 11:52:43 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 17C702639DB; Fri, 21 Oct 2022 08:52:39 -0700 (PDT) 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 561A661F12; 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> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 */ >