Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751476AbdINRxW convert rfc822-to-8bit (ORCPT ); Thu, 14 Sep 2017 13:53:22 -0400 Received: from mx2.suse.de ([195.135.220.15]:58686 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751352AbdINRxV (ORCPT ); Thu, 14 Sep 2017 13:53:21 -0400 Date: Thu, 14 Sep 2017 10:53:13 -0700 From: Davidlohr Bueso To: akpm@linux-foundation.org Cc: vbabka@suse.cz, parri.andrea@gmail.com, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH] mm,compaction: serialize waitqueue_active() checks (for real) Message-ID: <20170914175313.GB811@linux-80c1.suse> References: <20170914000908.713-1-dave@stgolabs.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: <20170914000908.713-1-dave@stgolabs.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1870 Lines: 58 On Wed, 13 Sep 2017, Davidlohr Bueso wrote: >- /* >- * Pairs with implicit barrier in wait_event_freezable() >- * such that wakeups are not missed in the lockless >- * waitqueue_active() call. >- */ > smp_acquire__after_ctrl_dep(); sorry, forgot to delete this. Here's v2. -----8<----------------------------------------------------- [PATCH v2] mm,compaction: serialize waitqueue_active() checks (for real) Andrea brought to my attention that the L->{L,S} guarantees are completely bogus for this case. I was looking at the diagram, from the offending commit, when that _is_ the race, we had the load reordered already. What we need is at least S->L semantics, thus simply use wq_has_sleeper() to serialize the call for good. Fixes: 46acef048a6 (mm,compaction: serialize waitqueue_active() checks) Reported-by: Andrea Parri Signed-off-by: Davidlohr Bueso --- mm/compaction.c | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) diff --git a/mm/compaction.c b/mm/compaction.c index fb548e4c7bd4..03d31a875341 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1999,17 +1999,14 @@ void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_idx) if (pgdat->kcompactd_max_order < order) pgdat->kcompactd_max_order = order; - /* - * Pairs with implicit barrier in wait_event_freezable() - * such that wakeups are not missed in the lockless - * waitqueue_active() call. - */ - smp_acquire__after_ctrl_dep(); - if (pgdat->kcompactd_classzone_idx > classzone_idx) pgdat->kcompactd_classzone_idx = classzone_idx; - if (!waitqueue_active(&pgdat->kcompactd_wait)) + /* + * Pairs with implicit barrier in wait_event_freezable() + * such that wakeups are not missed. + */ + if (!wq_has_sleeper(&pgdat->kcompactd_wait)) return; if (!kcompactd_node_suitable(pgdat)) -- 2.12.0