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 5E2BEFA373F for ; Fri, 21 Oct 2022 16:10:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230175AbiJUQKJ (ORCPT ); Fri, 21 Oct 2022 12:10:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52652 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230091AbiJUQKG (ORCPT ); Fri, 21 Oct 2022 12:10:06 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C0C6FF9876; Fri, 21 Oct 2022 09:10:04 -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 5A3FC61F0B; Fri, 21 Oct 2022 16:10:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2AA5C433D6; Fri, 21 Oct 2022 16:10:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1666368603; bh=NZzqyU+bobEWKzEadvtSYF/+VAx77MPAMZ3IBqV2Hl0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=LmM4Bmy9Tiahnfw2j7UouzsVbcjxZOVggWesE6VdAlwRXf0pXtTqcsoCw8kUHTtJM ijZIu9TAksJIEbTVuD7CXtD1427Pt15UIYLpp+qzo2Y3Fa8YCRcjo/TjxgZ4duGPVF 1vckVYgpYgfHjurtpZaPL3cBnsF4bGM8k4c9TnhMZ6Aoil+3uoKgwUibXl1NTDuXfi 5FIIv0PciLlyJYDCnllEwFvbh8CiZDL/HzuKQ+gImEOcTjuWXlGRiSsu8gbfIyFkYK RbTtav+ERW963sW//fHdaPpkw5nI2XYhYFYhCad72PkSUARwFNgCwK2z0d06guhX4f RQQzOsLPSnVHA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 5861B5C0543; Fri, 21 Oct 2022 09:10:03 -0700 (PDT) Date: Fri, 21 Oct 2022 09:10:03 -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: <20221021161003.GL5600@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> <20221021154224.GJ5600@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 21, 2022 at 05:50:39PM +0200, Vlastimil Babka wrote: > On 10/21/22 17:42, Paul E. McKenney wrote: > > 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 > > It was there :) One of those mornings, I guess... Thank you very much!!! Thanx, Paul > >> > 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 */ > >> >