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 F35A5C433FE for ; Thu, 20 Oct 2022 16:32:04 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 1258A8E0002; Thu, 20 Oct 2022 12:32:04 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0D51B8E0001; Thu, 20 Oct 2022 12:32:04 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F06088E0002; Thu, 20 Oct 2022 12:32:03 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id E13CE8E0001 for ; Thu, 20 Oct 2022 12:32:03 -0400 (EDT) Received: from smtpin26.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id B4FB91A0921 for ; Thu, 20 Oct 2022 16:32:03 +0000 (UTC) X-FDA: 80041869726.26.971A777 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf30.hostedemail.com (Postfix) with ESMTP id 18EF280025 for ; Thu, 20 Oct 2022 16:32:01 +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 ams.source.kernel.org (Postfix) with ESMTPS id 19947B828AE; Thu, 20 Oct 2022 16:32:00 +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> ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666283522; 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=26wwB9l8jZyjbsspbR1YHc0x6MH6I1F0/J4sg4MmqD8=; b=ikzuadbAzfKlYyVuZuovc3jMvE9PJuwbOnhzfzLrFbzZ10DqtIBGhz/MLUwUd6uEQ8aH1B vlgN/9stijboeRs2n6MYwF5Ku0T936Z3vaIjURZca0OvRCUaai+t56GChI/12eB+fGZP+k 93zwc56AYNf5BhdDvJ0IG21E5u1tB3Q= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=PIucaDp1; spf=pass (imf30.hostedemail.com: domain of "SRS0=HQ10=2V=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 145.40.68.75 as permitted sender) smtp.mailfrom="SRS0=HQ10=2V=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666283522; a=rsa-sha256; cv=none; b=0DoO5XbK/jvHrMgoKkIO7x2PQ7z5ASA8KWmUO6MgYokl3DtWXSeLwc128DdWHVa+esWelp MBuH4cmmMMRs82c3Gdo8SWSfeqZQsE2IadEQ/qXbKg4PPndPzND+AR9gBfcnZWeSXZTrcp Oby1bRuP7W3t3HppNIePuOLrPDEeuU0= Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=PIucaDp1; spf=pass (imf30.hostedemail.com: domain of "SRS0=HQ10=2V=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 145.40.68.75 as permitted sender) smtp.mailfrom="SRS0=HQ10=2V=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 18EF280025 X-Stat-Signature: ixcmp9qectm1ehoun7w9akg68z1gcxqw X-Rspam-User: X-HE-Tag: 1666283521-180924 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 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 */ >