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 2558FC43217 for ; Thu, 20 Oct 2022 16:32:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229556AbiJTQcD (ORCPT ); Thu, 20 Oct 2022 12:32:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229664AbiJTQcB (ORCPT ); Thu, 20 Oct 2022 12:32:01 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DF84B1AFABF; Thu, 20 Oct 2022 09:31:59 -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 7A62161C55; Thu, 20 Oct 2022 16:31:59 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C54E3C433C1; Thu, 20 Oct 2022 16:31:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1666283518; bh=m0e5s+7vjQK6i5I/82ZYyoeq25NadpHcQ6GXllTid18=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=PIucaDp1d5kUDRaubYsxusNCAuDLzpDrQg7XAAKUe9nrcLJT93KOsjevUkIYF6EmQ uu6qrTdk/2zLKxOZMPq/tl5OdS9BXAVTZI48XhBRneYRp4zaW0sbWipBMKjjs1BrLh PQbNGFIcoD5XNbZBR00fKVsinXtbd9/zzk8aJ6xBv2fhejhScTNAebmzYsGq10Zwst 81cWXGN+stTDtL4IHBG22EVq7UvxJd9yPEz9WhhJY0AjN9AITfEWgIfEoqDhZc6Hxs V/egd3ESGZWdk7C16Cre2i9NykIKIWec5ULpASlJNh0cwE+gJsG+7cp+Vt5A/eXvGQ xjrERUBjBE8WQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 62E865C04D0; Thu, 20 Oct 2022 09:31:58 -0700 (PDT) Date: Thu, 20 Oct 2022 09:31:58 -0700 From: "Paul E. McKenney" To: Vlastimil Babka Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, Christoph Lameter , 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: <20221020163158.GP5600@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221019224652.GA2499358@paulmck-ThinkPad-P17-Gen-1> <20221019224659.2499511-5-paulmck@kernel.org> <4a29b39a-b9b5-9c95-e43a-9e5f87801786@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4a29b39a-b9b5-9c95-e43a-9e5f87801786@suse.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 20, 2022 at 09:10:49AM +0200, Vlastimil Babka wrote: > On 10/20/22 00:46, 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. > > s/SLAB_DESTROY_BY_RCU/SLAB_TYPESAFE_BY_RCU/ in subject, commit log and the > added comment? :) Boy, I was certainly living in the past when I did this patch, wasn't I? Thank you, will fix on next rebase. > > Signed-off-by: Paul E. McKenney > > 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: > > --- > > include/linux/slab.h | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/include/linux/slab.h b/include/linux/slab.h > > index 90877fcde70bd..446303e385265 100644 > > --- a/include/linux/slab.h > > +++ b/include/linux/slab.h > > @@ -76,6 +76,12 @@ > > * 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_DESTROY_BY_RCU without first acquiring a reference > > + * as described above. The reason is that SLAB_DESTROY_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(). > > + * > > Wonder if slab caches with a constructor should be OK here as AFAIK it > should mean the object has to be in the initialized state both when > allocated and freed? It does look that way, thank you! And __i915_request_ctor(), sighand_ctor(), and anon_vma_ctor() actually do this, initializing a lock in the process. The ctor function could just initialize the locks, and all would be well. In addition, this makes sequence-lock-like approaches a bit easier, as in "just use a sequence lock". I will update with attribution. Thanx, Paul > > * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU. > > */ > > /* Defer freeing slabs to RCU */ >