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 X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9E940C433EF for ; Wed, 8 Sep 2021 02:53:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8464E61100 for ; Wed, 8 Sep 2021 02:53:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347426AbhIHCy4 (ORCPT ); Tue, 7 Sep 2021 22:54:56 -0400 Received: from mail.kernel.org ([198.145.29.99]:50398 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345895AbhIHCyg (ORCPT ); Tue, 7 Sep 2021 22:54:36 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id B749A61108; Wed, 8 Sep 2021 02:53:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1631069609; bh=/5hpTnM4B74sjwFaTICnFDfKetGiCwry56EIsgklH7o=; h=Date:From:To:Subject:In-Reply-To:From; b=XYM7Tds6CpCC4/0O86RDI94fopjzbkoq69VGRuRrxBolv2xqr5Pez6/zzD+wm7+KZ K98GyfATSzd+/ut4AtGi9qLxFwlagxisLoiCzaN/zjADoHNkmNscrq9T38xWqX6FKH VSwbAAb7tF9CyddmwtU+Y2G9s8lcZiVDnkI7m9+I= Date: Tue, 07 Sep 2021 19:53:28 -0700 From: Andrew Morton To: akpm@linux-foundation.org, bigeasy@linutronix.de, brouer@redhat.com, cl@linux.com, efault@gmx.de, iamjoonsoo.kim@lge.com, jannh@google.com, linux-mm@kvack.org, mgorman@techsingularity.net, mm-commits@vger.kernel.org, penberg@kernel.org, quic_qiancai@quicinc.com, rientjes@google.com, tglx@linutronix.de, torvalds@linux-foundation.org, vbabka@suse.cz Subject: [patch 010/147] mm, slub: simplify kmem_cache_cpu and tid setup Message-ID: <20210908025328.vBeOhEnkD%akpm@linux-foundation.org> In-Reply-To: <20210907195226.14b1d22a07c085b22968b933@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Vlastimil Babka Subject: mm, slub: simplify kmem_cache_cpu and tid setup In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee that our kmem_cache_cpu pointer is from the same cpu as the tid value. Currently that's done by reading the tid first using this_cpu_read(), then the kmem_cache_cpu pointer and verifying we read the same tid using the pointer and plain READ_ONCE(). This can be simplified to just fetching kmem_cache_cpu pointer and then reading tid using the pointer. That guarantees they are from the same cpu. We don't need to read the tid using this_cpu_read() because the value will be validated by this_cpu_cmpxchg_double(), making sure we are on the correct cpu and the freelist didn't change by anyone preempting us since reading the tid. Link: https://lkml.kernel.org/r/20210904105003.11688-11-vbabka@suse.cz Signed-off-by: Vlastimil Babka Acked-by: Mel Gorman Cc: Christoph Lameter Cc: David Rientjes Cc: Jann Horn Cc: Jesper Dangaard Brouer Cc: Joonsoo Kim Cc: Mike Galbraith Cc: Pekka Enberg Cc: Qian Cai Cc: Sebastian Andrzej Siewior Cc: Thomas Gleixner Signed-off-by: Andrew Morton --- mm/slub.c | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-) --- a/mm/slub.c~mm-slub-simplify-kmem_cache_cpu-and-tid-setup +++ a/mm/slub.c @@ -2882,15 +2882,14 @@ redo: * reading from one cpu area. That does not matter as long * as we end up on the original cpu again when doing the cmpxchg. * - * We should guarantee that tid and kmem_cache are retrieved on - * the same cpu. It could be different if CONFIG_PREEMPTION so we need - * to check if it is matched or not. + * We must guarantee that tid and kmem_cache_cpu are retrieved on the + * same cpu. We read first the kmem_cache_cpu pointer and use it to read + * the tid. If we are preempted and switched to another cpu between the + * two reads, it's OK as the two are still associated with the same cpu + * and cmpxchg later will validate the cpu. */ - do { - tid = this_cpu_read(s->cpu_slab->tid); - c = raw_cpu_ptr(s->cpu_slab); - } while (IS_ENABLED(CONFIG_PREEMPTION) && - unlikely(tid != READ_ONCE(c->tid))); + c = raw_cpu_ptr(s->cpu_slab); + tid = READ_ONCE(c->tid); /* * Irqless object alloc/free algorithm used here depends on sequence @@ -3164,11 +3163,8 @@ redo: * data is retrieved via this pointer. If we are on the same cpu * during the cmpxchg then the free will succeed. */ - do { - tid = this_cpu_read(s->cpu_slab->tid); - c = raw_cpu_ptr(s->cpu_slab); - } while (IS_ENABLED(CONFIG_PREEMPTION) && - unlikely(tid != READ_ONCE(c->tid))); + c = raw_cpu_ptr(s->cpu_slab); + tid = READ_ONCE(c->tid); /* Same with comment on barrier() in slab_alloc_node() */ barrier(); _