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=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham 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 373C9C432C1 for ; Tue, 24 Sep 2019 14:36:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 14668214AF for ; Tue, 24 Sep 2019 14:36:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2441390AbfIXOgV (ORCPT ); Tue, 24 Sep 2019 10:36:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54356 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388630AbfIXOgV (ORCPT ); Tue, 24 Sep 2019 10:36:21 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 91D81309BF1A; Tue, 24 Sep 2019 14:36:20 +0000 (UTC) Received: from t460s.redhat.com (ovpn-116-245.ams2.redhat.com [10.36.116.245]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0B2181001B08; Tue, 24 Sep 2019 14:36:15 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, David Hildenbrand , Andrew Morton , Oscar Salvador , Michal Hocko , Pavel Tatashin , Dan Williams , Thomas Gleixner Subject: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock Date: Tue, 24 Sep 2019 16:36:15 +0200 Message-Id: <20190924143615.19628-1-david@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Tue, 24 Sep 2019 14:36:20 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Since commit 3f906ba23689 ("mm/memory-hotplug: switch locking to a percpu rwsem") we do a cpus_read_lock() in mem_hotplug_begin(). This was introduced to fix a potential deadlock between get_online_mems() and get_online_cpus() - the memory and cpu hotplug lock. The root issue was that build_all_zonelists() -> stop_machine() required the cpu hotplug lock: The reason is that memory hotplug takes the memory hotplug lock and then calls stop_machine() which calls get_online_cpus(). That's the reverse lock order to get_online_cpus(); get_online_mems(); in mm/slub_common.c So memory hotplug never really required any cpu lock itself, only stop_machine() and lru_add_drain_all() required it. Back then, stop_machine_cpuslocked() and lru_add_drain_all_cpuslocked() were used as the cpu hotplug lock was now obtained in the caller. Since commit 11cd8638c37f ("mm, page_alloc: remove stop_machine from build all_zonelists"), the stop_machine_cpuslocked() call is gone. build_all_zonelists() does no longer require the cpu lock and does no longer make use of stop_machine(). Since commit 9852a7212324 ("mm: drop hotplug lock from lru_add_drain_all()"), lru_add_drain_all() "Doesn't need any cpu hotplug locking because we do rely on per-cpu kworkers being shut down before our page_alloc_cpu_dead callback is executed on the offlined cpu.". The lru_add_drain_all_cpuslocked() variant was removed. So there is nothing left that requires the cpu hotplug lock. The memory hotplug lock and the device hotplug lock are sufficient. Cc: Andrew Morton Cc: Oscar Salvador Cc: Michal Hocko Cc: Pavel Tatashin Cc: Dan Williams Cc: Thomas Gleixner Signed-off-by: David Hildenbrand --- RFC -> v1: - Reword and add more details why the cpu hotplug lock was needed here in the first place, and why we no longer require it. --- mm/memory_hotplug.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index c3e9aed6023f..5fa30f3010e1 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -88,14 +88,12 @@ __setup("memhp_default_state=", setup_memhp_default_state); void mem_hotplug_begin(void) { - cpus_read_lock(); percpu_down_write(&mem_hotplug_lock); } void mem_hotplug_done(void) { percpu_up_write(&mem_hotplug_lock); - cpus_read_unlock(); } u64 max_mem_size = U64_MAX; -- 2.21.0