From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id A3ED96B78C5 for ; Thu, 6 Dec 2018 02:33:02 -0500 (EST) Received: by mail-ed1-f70.google.com with SMTP id c53so11243309edc.9 for ; Wed, 05 Dec 2018 23:33:02 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id k7si3378331edb.132.2018.12.05.23.33.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 05 Dec 2018 23:33:01 -0800 (PST) Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wB67RgFW050577 for ; Thu, 6 Dec 2018 02:32:59 -0500 Received: from e06smtp01.uk.ibm.com (e06smtp01.uk.ibm.com [195.75.94.97]) by mx0b-001b2d01.pphosted.com with ESMTP id 2p6yd5r6ys-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 06 Dec 2018 02:32:59 -0500 Received: from localhost by e06smtp01.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 6 Dec 2018 07:32:57 -0000 Date: Thu, 6 Dec 2018 09:32:51 +0200 From: Mike Rapoport Subject: Re: [PATCH v2 2/2] core-api/memory-hotplug.rst: divide Locking Internal section by different locks References: <20181205023426.24029-1-richard.weiyang@gmail.com> <20181206002622.30675-1-richard.weiyang@gmail.com> <20181206002622.30675-2-richard.weiyang@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181206002622.30675-2-richard.weiyang@gmail.com> Message-Id: <20181206073250.GI19181@rapoport-lnx> Sender: owner-linux-mm@kvack.org List-ID: To: Wei Yang Cc: david@redhat.com, mhocko@suse.com, osalvador@suse.de, akpm@linux-foundation.org, linux-doc@vger.kernel.org, linux-mm@kvack.org On Thu, Dec 06, 2018 at 08:26:22AM +0800, Wei Yang wrote: > Currently locking for memory hotplug is a little complicated. > > Generally speaking, we leverage the two global lock: > > * device_hotplug_lock > * mem_hotplug_lock > > to serialise the process. > > While for the long term, we are willing to have more fine-grained lock > to provide higher scalability. > > This patch divides Locking Internal section based on these two global > locks to help readers to understand it. Also it adds some new finding to > enrich it. > > [David: words arrangement] > > Signed-off-by: Wei Yang Reviewd-by: Mike Rapoport > --- > v2: adjustment based on David and Mike comment > --- > Documentation/core-api/memory-hotplug.rst | 27 ++++++++++++++++++++++++--- > 1 file changed, 24 insertions(+), 3 deletions(-) > > diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst > index de7467e48067..51d477ad4b80 100644 > --- a/Documentation/core-api/memory-hotplug.rst > +++ b/Documentation/core-api/memory-hotplug.rst > @@ -89,6 +89,20 @@ NOTIFY_STOP stops further processing of the notification queue. > Locking Internals > ================= > > +In addition to fine grained locks like pgdat_resize_lock, there are three locks > +involved > + > +- device_hotplug_lock > +- mem_hotplug_lock > +- device_lock > + > +Currently, they are twisted together for all kinds of reasons. The following > +part is divided into device_hotplug_lock and mem_hotplug_lock parts > +respectively to describe those tricky situations. > + > +device_hotplug_lock > +--------------------- > + > When adding/removing memory that uses memory block devices (i.e. ordinary RAM), > the device_hotplug_lock should be held to: > > @@ -111,13 +125,20 @@ As the device is visible to user space before taking the device_lock(), this > can result in a lock inversion. > > onlining/offlining of memory should be done via device_online()/ > -device_offline() - to make sure it is properly synchronized to actions > -via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type) > +device_offline() - to make sure it is properly synchronized to actions via > +sysfs. Even if mem_hotplug_lock is used to protect the process, because of the > +lock inversion described above, holding device_hotplug_lock is still advised > +(to e.g. protect online_type) > + > +mem_hotplug_lock > +--------------------- > > When adding/removing/onlining/offlining memory or adding/removing > heterogeneous/device memory, we should always hold the mem_hotplug_lock in > write mode to serialise memory hotplug (e.g. access to global/zone > -variables). > +variables). Currently, we take advantage of this to serialise sparsemem's > +mem_section handling in sparse_add_one_section() and > +sparse_remove_one_section(). > > In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read > mode allows for a quite efficient get_online_mems/put_online_mems > -- > 2.15.1 > -- Sincerely yours, Mike.